feat(scripts): hold every section landing index block to its meta.json (#10738)

Adds check-section-landing-index.mjs. A section's meta.json is the routing
source fumadocs builds the sidebar from; the landing page's
"## What's in this module" block is hand-kept beside it and nothing read the
two against each other, so it drifted in both directions one edit at a time
with every gate green (#9604 runtime-services, #10339 permissions,
#10737 ai).

The heading is the opt-in, not the rendering shape: 8 sections declare it,
4 as bullet lists and 4 as Card grids, and the gate reads both identically.
26 landing pages that declare no index block are not held.

Committed before the docs fixes so the red is reproducible at this commit.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SPBxLsqQGCVL5z5UXvgipH

Author: claude