sync-starlight.mjs anchor mapping drops ###-level headings, producing silent 404 cross-links in mirrors

[theagenticguy]

Problem

sync-starlight.mjs maps only ##-level USER_GUIDE.md anchors when rewriting cross-links for the Starlight mirrors. A ###-level heading falls through the mapping, and the mirror rewrites the cross-link to a page that does not contain that anchor. The result is a silent 404: the page loads, the anchor jump fails, and astro check cannot detect it because the link target page exists.

Why this is a design issue, not a per-link fix

This is the third instance of the anchor/route-mapping bug class in this stack:

1. COST_ATTRIBUTION link mapping
2. #repository-onboarding anchor
3. The ### heading caught in self-review on docs(cost): document every max_budget_usd surface and reconcile the Blueprint gap #763 (relinked to #per-repo-overrides as a workaround)

Three instances of the same class point at the mapping design rather than the individual links. Candidate directions:

• Map anchors at all heading levels, not just ##
• Add a post-sync link-check step that resolves every rewritten anchor against the generated pages and fails the sync on a miss (closing the gap astro check leaves)

Origin

Found during self-review of PR #763 — see the merge-guidance comment: #763 (comment)