feat(spec): derive reference page descriptions from the module doc block

build-docs.ts wrote every generated module page as "<Title> protocol
schemas" and every category overview as "Complete reference for all
<title> schemas" — 210 of 211 generated pages under the 70 characters a
search result keeps. The new lib/page-description.ts reads the same
module doc block the page body opens with and condenses its lead into
70-160 characters; a thin lead is completed with the page's schema
names, and a module with no doc block gets a sentence naming them. The
value is emitted double-quoted so prose punctuation cannot change the
YAML. content/docs/references regenerated with gen:docs.

Claude-Session: https://claude.ai/code/session_01RCEEP3Z95jrpXCeF3it3Y2
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude