fix(ext/sw): null gitHost on explicit lock (Plan C Phase 6)

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>

extension/src/service-worker/router/popup-only.ts

@@ -59,6 +59,9 @@ export async function handle(
     case 'lock':
       session.clearCurrent();
       state.manifest = null;
+      // Don't leak the cached git-host (incl. lastSyncAt) across a lock —
+      // symmetric with the session-expiry path (index.ts); unlock rebuilds it.
+      state.gitHost = null;
       return { ok: true };
 
     case 'list_items': {