The explicit lock message handler nulled state.manifest but left
state.gitHost (now carrying the cached lastSyncAt) intact, so a lock then
re-unlock within one service-worker lifetime surfaced a stale sync time.
Null gitHost here too — symmetric with the session-expiry path (index.ts)
and completing Plan C Phase 5's don't-leak-git-host-across-a-lock intent;
unlock rebuilds it on demand.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>