docs(design): add DESIGN.md design system documentation

DESIGN.md

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+# Docs Design System
+
+This file documents **design intent and conventions** for `docs.mergify.com` — the "why" behind
+decisions, not the values themselves. For exact values (colors, spacing, typography), read the
+source files listed below; the code is the source of truth.
+
+> Companion file to `CLAUDE.md`: `CLAUDE.md` covers project setup, commands, and codebase rules.
+> `DESIGN.md` is the design system this site is built from. Keep them aligned: when a design rule
+> changes here, update the matching section in `CLAUDE.md` (or vice-versa) so they don't drift.
+
+## Source Files
+
+| What | Where |
+| --- | --- |
+| Primitive tokens (gray scale + product palette) | [`src/styles/tokens.css`](./src/styles/tokens.css) |
+| Semantic tokens + dark-mode remaps | [`src/styles/theme.css`](./src/styles/theme.css) |
+| Typography utility classes | [`src/styles/typography.css`](./src/styles/typography.css) |
+| Global content rules + section-accent system | [`src/styles/index.css`](./src/styles/index.css) |
+| Product accent colors (canonical source) | [`mergify.com/DESIGN.md`](../mergify.com/DESIGN.md) — "Product Accent Colors" section |
+| Reusable Astro components | [`src/components/`](./src/components/) |
+| Page layout templates | [`src/layouts/`](./src/layouts/) |
+| Icons (Phosphor bold + custom SVGs) | Inline `?raw` imports; no icon component abstraction yet |
+| Agent instructions | [`AGENTS.md`](./AGENTS.md) — "Design System" section |
+| Claude instructions | [`CLAUDE.md`](./CLAUDE.md) — "Design System" section |
+
+## Visual Theme
+
+`docs.mergify.com` is a reference site for developers who are already using or evaluating Mergify.
+They arrive from search, GitHub links, and product UI. The goal is to answer their question quickly
+and get out of the way. It is **not** a conversion funnel — that is `mergify.com`'s job.
+
+Built with Astro 5 + MDX + plain CSS. No Tailwind (the site predates the Tailwind migration on the
+marketing site; adding it is out of scope). Dark mode is supported via the `:root.theme-dark`
+class-based mechanism.
+
+**Key characteristics:**
+
+- Neutral foundation — primary text and buttons are `gray-900` / `gray-50` (black/white), not
+  `--color-mergify-blue`. Product palette colors appear only when a surface is *about* that product.
+- Clean gray ramp — intentionally diverges from `mergify.com`'s gray scale, which has
+  `gray-50 == gray-200` collisions. The docs ramp has no duplicates; `gray-50` through `gray-900`
+  are distinct values you can reach for without reading a gotchas table.
+- Light + dark — the marketing site is light-only; docs ships both modes because developers working
+  in dark terminals expect it.
+- Inter variable font, same as the marketing site. Self-hosted via `@fontsource/inter`.
+- Compact layout — content shares horizontal space with a left sidebar and right ToC. The hero
+  typography cap is smaller than marketing's (`clamp(2.5rem, 4.5vw, 3rem)` vs. 64px) because the
+  reading column is narrower.
+
+## Color Tokens
+
+All colors live in three layers. **Components consume only the semantic layer.**
+
+### Layer 1 — Primitive (`tokens.css`)
+
+Fixed hex values. Never changes between light and dark mode. Never referenced directly by
+components or content rules.
+
+```css
+/* Neutrals */
+--color-white, --color-dark
+--color-gray-50 … --color-gray-900
+
+/* Product palette — verbatim from mergify.com */
+--color-teal-400, --color-teal-700
+--color-purple-400, --color-purple-700
+/* …etc for orange, blue, coral, rose… */
+--color-blue-800  /* docs-only: link hover */
+```
+
+The two exceptions where primitive tokens are allowed:
+
+1. Inside `tokens.css` itself (where they are defined).
+2. Inside `theme.css` (where they are mapped to semantic names).
+
+Nowhere else.
+
+### Layer 2 — Semantic (`theme.css`)
+
+Mode-aware names that components and content rules consume. Defined once for light, remapped once
+in `:root.theme-dark`. These are the **only** tokens components should touch.
+
+```css
+--theme-text            /* primary text */
+--theme-text-secondary  /* supporting text */
+--theme-text-muted      /* de-emphasized text, captions */
+--theme-bg              /* page background */
+--theme-bg-content      /* card / content surface */
+--theme-bg-offset       /* raised surface (inline code bg, sidebar) */
+--theme-border          /* borders, dividers */
+--theme-link            /* link color */
+--theme-link-hover      /* link hover */
+--theme-accent          /* primary action / heading emphasis */
+```
+
+### Layer 3 — Section accents (`index.css`)
+
+A `--section-accent` custom property set on `<body>` by class. Each documentation section gets
+its own accent color; the ToC active-link and sidebar hover states consume it automatically. One
+rule block, not one block per section.
+
+```css
+body { --section-accent: var(--theme-link); }
+body.section-merge-queue { --section-accent: var(--color-teal-700); }
+/* …etc… */
+```
+
+### The discipline rule
+
+**Components never reference primitive tokens directly.** If you find yourself writing
+`var(--color-gray-300)` inside a component, stop and use `var(--theme-border)` instead. The
+primitive layer exists so `theme.css` has legible, self-documenting mappings — not so consumers
+can bypass the semantic layer.
+
+## Product Accent Colors
+
+Product colors are verbatim from `mergify.com/DESIGN.md`. The docs site reuses the same palette
+without adding new tokens.
+
+| Product | Dark / Label | Light / Accent | Hex (label) |
+| --- | --- | --- | --- |
+| Merge Queue | `--color-teal-700` | `--color-teal-400` | #1CB893 |
+| CI Insights | `--color-purple-700` | `--color-purple-400` | #4D59E0 |
+| Test Insights | `--color-orange-700` | `--color-orange-400` | #F27B2A |
+| Merge Protections | `--color-blue-700` | `--color-blue-400` | #43A7E5 |
+| Stacks | `--color-coral-700` | `--color-coral-400` | #E53935 |
+| Workflow Automation | `--color-rose-700` | `--color-rose-400` | #E61E71 |
+
+One docs-only addition: `--color-blue-800` (#237caf) for link hover. It is a darker shade of
+`blue-700` and exists only because docs is text-heavy with many cross-references; the marketing
+site uses product colors for links far less often.
+
+### Section-accent body classes
+
+The `<body>` element receives a class based on the active documentation section. This drives the
+ToC active-link color and left-sidebar hover highlight.
+
+| Body class | Section | Accent token |
+| --- | --- | --- |
+| (no class) | General / configuration | `--theme-link` (blue-700) |
+| `section-merge-queue` | Merge Queue | `--color-teal-700` |
+| `section-ci-insights` | CI Insights | `--color-purple-700` |
+| `section-test-insights` | Test Insights | `--color-orange-700` |
+| `section-merge-protections` | Merge Protections | `--color-blue-700` |
+| `section-stacks` | Stacks | `--color-coral-700` |
+| `section-workflow` | Workflow Automation | `--color-rose-700` |
+
+The class is applied by the layout; do not hardcode accent colors inside section-specific
+components. Consume `var(--section-accent)` and `var(--section-accent-bg)` instead.
+
+Note on link color vs. Merge Protections accent: both use `blue-700`. On a Merge Protections page,
+inline links and product pills share a hue but serve different roles (underlined inline link vs.
+solid pill). The visual conflation is minor and preferable to introducing a new shade.
+
+## Dark Mode
+
+### Mechanism
+
+`:root.theme-dark` is added to the `<html>` element by a `<script is:inline>` in `HeadCommon.astro`
+before the first paint, based on `localStorage` or `prefers-color-scheme`. The `ThemeToggleButton`
+updates both `localStorage` and the class at runtime. No JavaScript framework is involved; the
+class toggle is pure DOM.
+
+### Where remaps live
+
+Every dark-mode remap is in one place: the `:root.theme-dark` block in `theme.css`. Nowhere else.
+
+```
+tokens.css         ← primitives (fixed, no dark remap)
+theme.css          ← semantic layer
+  :root            ← light semantic values
+  :root.theme-dark ← dark overrides (ONLY here)
+typography.css     ← utility classes (mode-agnostic)
+index.css          ← global rules, section accents (no mode-conditional CSS)
+```
+
+Component-scoped styles never include `:root.theme-dark` blocks. They consume semantic tokens and
+dark mode works automatically.
+
+### Product palette does not flip
+
+`--color-teal-700`, `--color-purple-700`, and all other product tokens stay the same value in dark
+mode. Brand recognition and in-context UI legibility depend on these colors being stable; the
+surrounding surface contrast changes instead.
+
+### Adding a new token
+
+1. Add the primitive value to `:root` in `tokens.css`.
+2. Add a semantic name in the `:root` block in `theme.css`, pointing at the primitive.
+3. Add a dark-mode override in the `:root.theme-dark` block in `theme.css`, also pointing at a
+   primitive (or a different primitive).
+4. Document the new token in this file under the appropriate section.
+5. Consume the semantic token — never the primitive — in components.
+
+## Typography
+
+Two font families:
+
+- **Inter variable** — everything visible to users. Loaded via `@fontsource/inter` (400, 500,
+  600, 700 weights).
+- **Poppins** — loaded but limited to decorative use (100, 200 weights). Prefer Inter for all
+  body and heading copy.
+- **System mono** — code blocks and inline code.
+
+### Utility classes
+
+Defined in `typography.css`. Apply by class on custom-built pages and Astro components.
+MDX-rendered headings automatically get the same sizing via `#main-content h1..h6` selectors in
+`index.css` — authors writing MDX do not need to add classes.
+
+| Class | Size | Weight | Line-height | Use |
+| --- | --- | --- | --- | --- |
+| `heading-hero` | clamp(2.5rem, 4.5vw, 3rem) | 500 | 1.2 | Homepage / hub-page H1. Smaller cap than marketing (64px) because the content column is narrower. |
+| `heading-page` | 2rem | 500 | 1.2 | Regular doc-page H1. |
+| `heading-section` | 1.5rem | 500 | 1.3 | Content H2. |
+| `heading-subsection` | 1.25rem | 500 | 1.3 | Content H3. |
+| `heading-detail` | 1.125rem | 500 | 1.4 | Content H4. |
+| `heading-minor` | 1rem | 500 | 1.4 | H5 / H6. |
+| `text-subtitle` | 1.125rem | 400 | 1.5 | Page description / lead paragraph. |
+| `text-eyebrow` | 0.75rem uppercase | 600 | — | Small label above section headings. Rarely used in prose docs; reserved for hub-page heroes (B-pass). |
+
+Letter-spacing: `-0.025em` on `heading-hero`, `heading-page`, `heading-section`. `-0.01em` on
+`heading-subsection`. Not set on `heading-detail` and smaller. All headings use
+`text-wrap: balance`.
+
+### When to use `heading-hero` vs `heading-page`
+
+`heading-hero` is for the homepage and product hub pages (`/merge-queue`, `/ci-insights`, etc.)
+where the H1 is a marketing-register statement that needs visual weight. `heading-page` is for
+every regular reference page — configuration options, how-to guides, API docs. When in doubt, use
+`heading-page`.
+
+### MDX content auto-styling
+
+Authors writing MDX do not add classes. The `#main-content h2` selector in `index.css` mirrors
+the `heading-section` properties; `#main-content h3` mirrors `heading-subsection`; and so on
+through H6. When you change a utility's properties, update the corresponding `#main-content`
+selector too — they must stay in sync.
+
+### Eyebrow rules
+
+`text-eyebrow` is uppercase, letter-spaced, small. Keep eyebrow labels to 2–4 words. Use the
+section's accent color (`var(--section-accent)`) when the eyebrow labels a product feature;
+use `var(--theme-text-muted)` for neutral sections. Eyebrows are currently used only on the
+homepage and hub pages — do not add them to prose documentation pages.
+
+## Layout & Spacing
+
+The layout is a classic three-column docs shell: left sidebar, content column, right ToC. Key
+sizing variables live in `theme.css`.
+
+| Variable | Value | Notes |
+| --- | --- | --- |
+| `--max-width` | 46em | Reading column max-width. Keeps prose lines short for readability. |
+| `--theme-sidebar-width` | ~18rem | Left sidebar width (see `theme.css` for exact breakpoint rules). |
+| `--theme-navbar-height` | 4rem | Sticky top nav height. Used in `calc()` for scroll offsets and sticky positioning. |
+| `--theme-mobile-toc-height` | varies | Height of the on-page mobile ToC bar, used in gradient calculations. |
+
+Do not add new `max-width` values without updating this table. The 46em reading column is
+intentional — narrower than the marketing site's 1200px container because docs is prose-heavy.
+
+## Components
+
+Before building anything custom, check `src/components/`. The components below are the primary
+building blocks; reach for them before rolling your own.
+
+| Component | Purpose |
+| --- | --- |
+| `Aside` | Callout boxes — `note`, `tip`, `caution`, `danger` variants. Use directive syntax in MDX: `:::note`. |
+| `Button` | Anchor or button element with `primary` (solid) and `outline` variants. Consumes product palette via `colorScheme` prop. |
+| `DocsetGrid` | Grid of doc-section cards, used on the homepage and hub pages. |
+| `CommunityButton` | "Join Slack" / community CTA button with Discord/Slack icon. |
+| `Breadcrumbs` | Page breadcrumb trail, rendered above the page title. |
+| `PageContent` | Content column wrapper — handles padding, max-width, ToC anchor injection. |
+| `Header` | Top navigation bar including the theme toggle and search trigger. |
+| `LeftSidebar` | Navigation sidebar with collapsible section groups. |
+| `RightSidebar` | On-page ToC with active-link tracking. |
+| `PageFeedback` | "Was this helpful?" widget at the bottom of every doc page. |
+
+Astro component files live in `src/components/`. Layout wrappers live in `src/layouts/`.
+
+## Voice
+
+The docs site serves **the searcher**, not the funnel. A developer who lands on a configuration
+reference page needs an accurate, direct answer — not brand personality.
+
+**Docs voice is informational + on-voice.** It is clear, neutral, and technically precise. It does
+not use Mergify's marketing voice ("fun + arrogant"). Marketing's confident-cocky register belongs
+on `mergify.com` product pages; on docs it reads as noise that buries the answer.
+
+Concretely:
+
+- Write for someone who arrived from a search query and needs to know what a config key does.
+- Skip openers like "Great news!" or "Simply add…". Cut to the content.
+- Do not editorialize ("This is the recommended way…") unless there is a genuine reason to steer
+  the reader (safety, correctness, performance).
+- Use second person ("you") consistently. Avoid the royal "we".
+- Code samples should be minimal and runnable. Do not add features to the example that the text
+  does not explain.
+
+For marketing-register copy (homepage hero, hub-page openers), use the `mergify-internal:writing-style`
+skill — it enforces the marketing voice conventions. For all prose documentation, use the
+`proofread-style` skill — it enforces the docs-specific rules above and catches AI-generated
+patterns (banned words: "leverage", "robust", "seamlessly", em-dashes used as filler, etc.).
+
+**Title case** for page titles and section headings. Sentence case for body copy. Capitalize the
+first part of a hyphenated compound modifier ("Auto-retry", not "Auto-Retry").
+
+## Code Rules (STRICT)
+
+These rules apply when writing or generating component, style, or page code.
+
+### Use semantic tokens
+
+- **Colors**: always `var(--theme-*)` or `var(--section-accent)` in component and content CSS.
+  `var(--color-*)` primitives are only allowed inside `tokens.css` and `theme.css`.
+- **Inline SVG `fill`/`stroke`**: the only place raw hex is permitted outside `tokens.css`. Use
+  `currentColor` wherever possible; fall back to a `var(--color-*)` primitive only when the SVG
+  must carry its own color independent of the theme.
+- **Typography**: use the utility classes (`heading-section`, `text-subtitle`, etc.) on custom
+  pages. Do not compose ad-hoc `font-size` / `font-weight` / `letter-spacing` combos from scratch.
+- **Dark mode**: never add a `:root.theme-dark` block inside a component-scoped `<style>`. Use
+  semantic tokens and let `theme.css` handle the remap automatically.
+
+### Do not use
+
+- **Hex literals** in any file except `tokens.css` and inline SVG `fill`/`stroke`.
+- **`--chakra-colors-*`** — the Chakra color system was never the design system; all remaining
+  references are migration debt to be removed.
+- **`--color-mergify-blue`** — retired. Use `var(--theme-link)` for link affordances or
+  `var(--theme-accent)` for primary surfaces.
+- **Per-component `:root.theme-dark` rules** — all dark remaps live in `theme.css`.
+- **Ad-hoc heading CSS** — no `h2 { font-size: 1.5rem; font-weight: 500; }` rules scattered
+  across component files. Use the typography utilities.
+
+### Self-correction list
+
+If you generate any of the following, **fix it immediately**:
+
+- Hex literal in a component or content rule (e.g., `color: #1cb893`) → find the matching
+  `--color-*` primitive, then replace with the appropriate `--theme-*` semantic token.
+- `var(--chakra-colors-*)` anywhere outside `theme.css` → replace with the semantic token
+  (see the migration table in Phase 3 of the implementation plan).
+- `var(--color-mergify-blue*)` anywhere → replace with `var(--theme-link)` or `var(--theme-accent)`.
+- `:root.theme-dark { … }` inside a `<style>` block in a component → move the logic to a semantic
+  token in `theme.css`.
+- `font-size: 1.5rem; font-weight: 500;` heading ad-hoc combo → use `heading-section` class.
+
+No exceptions unless the user explicitly overrides.
+
+## Future / Open Questions
+
+- **Phosphor icon migration.** The docs site uses a mix of Octicons and inline SVGs. The marketing
+  site standardized on Phosphor bold. Migrating docs to Phosphor is a separate scope — when
+  undertaken, update the "Source Files" table and the "Components" section above.
+- **B-pass homepage rework.** The homepage (`src/content/docs/index.mdx`) is a documentation page
+  masquerading as a landing page. B-pass will add a visual hero, one primary CTA per product,
+  and remove the duplicate Reference grid. Class swaps (`content-title` → `heading-hero`, etc.)
+  are landed in A-pass; the visual design work is deferred.
+- **C-pass reading experience.** Several UX issues deferred from A: the top nav disappears below
+  1392px (pick a smaller breakpoint or move links to an overflow menu), hub-page heroes (adopt the
+  eyebrow + product-color H1 pattern from marketing's `<ProductHero>`), right-sidebar empty state
+  on short pages (sticky "ask a question" CTA or hide entirely), h2 auto-numbering review (keep on
+  tutorials only or remove globally).
+- **Token sharing with the dashboard.** The dashboard has its own Figma-derived token set that
+  diverges from both marketing and docs. Alignment across all three surfaces is a longer-term
+  design-system project, not scoped to any single pass.