docs(design): rewrite DESIGN.md for library-first shell

[graydawnc]

What

Realign DESIGN.md with the library-first product that shipped in PRs #122–#133 (project identity, library shell, sidebar, project view, pin migration, ⌘K overlay, session detail). The doc had been frozen on the older "centered search box is the entire product" framing and explicitly labeled the home state as "transitional, will be replaced by Project View in PR 4" — PR 4 has long since shipped.

Why

DESIGN.md is the source of truth for visual decisions and is referenced by CLAUDE.md ("Always read DESIGN.md before making any visual or UI decisions"). Leaving it inconsistent with the shipped UI silently invites regressions: any new contributor reading it would build the wrong product. Recent decisions (Pin replacing Star, ⌘K overlay, no centered hero) also had no entry in the decisions log, so the why behind the pivot was undocumented.

How it connects

Pure docs change — no code, no asset, no behavior touched. Sets up the language used by the next two follow-ups: rewriting README.md + packages/landing/pages/docs/* for the new positioning (#135), and redesigning the landing page hero/specimen.

What changed

• Product Context: "local search engine for your thinking" → "local AI session library"; peers updated (Perplexity → DevonThink); positioning reframed around the shell, not the search box.
• Layout Philosophy: dropped "transitional" / "search-first home" language; added entries for Library Home, Project View, Session Detail, and the ⌘K overlay; alignment rule flipped from "centered on home" to "left-aligned everywhere".
• Type Scale: removed the 48px Logo (home) row; renamed search-input rows to reflect their new homes (overlay vs. results page).
• Spacing: density line and search-input padding line updated to drop "utility tool" + "home / results bar" framing.
• Motion: replaced the home → results translate animation with overlay open/close motion; main-pane state changes now explicitly instant.
• UI States: replaced the Search Bar block with Search (⌘K overlay); removed Source Chips (home screen) (gone); added new blocks for Library Home, Project View, and Pin Button.
• AI Search: agent selector and mode toggle now live in the overlay, not the results topbar.
• First-Person Language: Do/Don't example "You starred this" → "You pinned this" to match the Pin migration.
• Anti-patterns: third entry reversed direction — centered hero is now disallowed shell-wide; the ⌘K overlay is the only centered surface.
• Decisions Log: 3 new 2026-04-30 entries (library-first shell, Pin replaces Star, ⌘K overlay), with the first explicitly noting it reverses the 2026-03-27 "search box is the product" decision. Old 2026-03-27 entries kept on purpose so the reversal is legible.

Verification

• ✅ Re-read DESIGN.md end-to-end. Final residue scan (search.box|search bar|centered.*home|results.bar|home state|transitional|Star) returns only intentional hits: the explicit anti-pattern lines, the historical 2026-03-27 decisions log entries, and the new 2026-04-30 entries that reference them.
• ✅ Cross-checked the new sections against shipped components — Library Home matches LibraryLanding.tsx (PINNED + RECENT, date buckets, global listPinnedSessions); Project View matches ProjectView.tsx (Recent/Oldest/Most messages/Title sort, source filter chips, PINNED segment); Search (⌘K overlay) matches SearchOverlay.tsx (Scope = 'all' | 'project', Fast/AI mode toggle); Sidebar matches Sidebar.tsx after feat(app): sort sidebar projects #133's sort-menu addition; Pin Button matches PinButton.tsx.
• ✅ Decisions log row "Library-first shell replaces centered search home" closes with "Reverses the 2026-03-27 'search box is the product' decision" and sits directly under that 03-27 entry, so the reversal is legible without cross-referencing.