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

## 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 #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: https://github.com/aws-samples/sample-autonomous-cloud-coding-agents/pull/763#issuecomment-5289889688

sync-starlight.mjs から始め、Starlight ミラーで使われているアンカー/ルートのマッピングを調べ、続いて COST_ATTRIBUTION、#repository-onboarding、PR #763 に関連するケースを確認してください。完了条件は、マッピング設計が報告された見出しレベルのケースに対応するか、または sync が生成されたページに存在しないアンカーを持つ書き換え済みリンクを検出して拒否することです。

索引モデルが issue の本文から書いたものです。