v2.61.4

Make the site visible to LLMs and AI agents (#547)

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Implements LLM visibility for public pages (`/` and `/music`):
registry-driven sitemap, `/llms.txt`, `.md` twins, Accept negotiation,
and robots rules.

Closes [DG-224](https://linear.app/dgattey/issue/DG-224)

### What changed?

- Single `markdownPages` registry in `@dg/shared-core/routes/app`
- Page-local Markdown modules (`homepageMarkdown.ts`,
`musicMarkdown.ts`)
- Proxy Accept negotiation + internal `/llm-markdown` route
- Rebased onto main (#549 light-dark, #550 modern CSS)
- **Fix:** `ContentCard` no longer defaults `verticalSpan={1}` (was
clipping intro RichText)
- Removed visually hidden Markdown hint shell
- **Removed** PR screenshots workflow + config (resource-heavy,
inaccurate)

### Owner follow-up (CodeQL)

CodeQL is GitHub **default setup** (not a repo workflow). This agent
cannot disable it (API 403). To finish removing it:

1. Repo **Settings → Code security → Code scanning** → turn off CodeQL
default setup
2. Repo **Settings → Rules → Default branch** → remove required status
check `CodeQL` and the `code_scanning` rule (also remove `📸 Screenshots`
if still listed)

### Release info

- [x] Patch
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Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Dylan Gattey <dgattey@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>