Propagate F-010 auto-derived descriptions into llms.txt entries

[Aliserag]

Context

Phase 6.2 F-010 added an auto-derive helper in src/routes/docs/$.tsx that pulls a meta description from the first non-heading paragraph when frontmatter description: is absent. This fixed 13 docs pages with generic meta descriptions (T8 verified: 20/20 unique now).

Gap

The same auto-derive logic isn't reused in src/routes/llms[.]txt.ts. Result: only ~15 of 93 /llms.txt entries have a description suffix (only those with explicit frontmatter description). The other 78 entries are bare - [Title](url) lines.

R3 best-in-class rerun flagged this — fumadocs.dev's llms.txt has 100% description coverage; ours is at ~16%.

Fix

~5 LOC change: extract deriveDescription(processedMarkdown) from src/routes/docs/$.tsx into src/lib/derive-description.ts and reuse in both llms[.]txt.ts and llms-full[.]txt.ts. The collectEntries() walk already reads the raw MDX content via gray-matter; just add the fallback when frontmatter description is absent.

Verification

After fix: count of /llms.txt lines containing : (description suffix) should jump from ~15 to ~85+.

Why deferred

Out of scope for Phase 6.2 — was R3-rerun's only remaining P2 finding after the SHIP AS-IS verdict.