Harden local docs sync against dirty sibling checkouts

Migrated from: https://github.com/jeremi/registry-docs/issues/71
Original author: @jeremi
Source repository: `jeremi/registry-docs`
Source issue: `#71`
Source labels: enhancement, post-1.0
Source milestone/release intent: _none_
Target area/path: `docs/site`

## Issue

Audit candidate: CAND-ARTIFACTS-01, downgraded to local tooling hygiene.

Background:
GitHub Pages builds use clean runners and pinned refs, so this is not currently a public docs publication risk. Local developer builds can still prefer an existing sibling checkout without checking cleanliness or HEAD, which can confuse local review or generated archives.

Acceptance criteria:
- Local docs sync warns, rejects, or ignores sibling checkouts that are dirty or not at the pinned ref.
- GitHub CI behavior remains pinned-clean and does not depend on local sibling paths.
- Tests or script checks cover dirty and wrong-ref sibling checkout cases.

Non-goals:
- Changing public GitHub Pages deployment if it already builds from clean pinned refs.

## Migration Metadata

- Migrated to the public monorepo on 2026-06-25.
- Source title, body, labels, milestone/release intent, and pre-migration discussion were preserved where available.
- Code-grounded audit note: Docs sync still prefers sibling checkouts and lacks dirty/wrong-ref gates.
- Private/internal references, secrets, and obvious deployment-only details were redacted instead of copied forward.

Beginne in docs/site und finde den Einstiegspunkt für die lokale Docs-Synchronisierung; untersuche, wie benachbarte Checkouts ausgewählt werden und wie gepinnte refs dargestellt werden. Füge Prüfungen oder Warnungen für nicht saubere und auf den falschen ref zeigende benachbarte Checkouts hinzu, während das saubere CI-Verhalten mit gepinnten refs erhalten bleibt, und decke beide Fälle mit Tests oder Skriptprüfungen ab.

Vom Indexierungsmodell aus dem Issue-Text verfasst.