feat(ext/sw): get_vault_status handler + cached sync state (Plan C Phase 6)

Returns cached ahead/behind/lastSyncAt from the GitHost plus a live count of
active (non-trashed) manifest items. No network call — sync is user-initiated;
the sync handler records lastSyncAt (unix seconds). ahead/behind stay 0 in the
extension (writes go straight to the host, no local commit graph) and exist
for parity with relicario status.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -130,6 +130,8 @@ export async function handle(
       const handle = session.getCurrent();
       if (!handle || !state.gitHost) return { ok: false, error: 'vault_locked' };
       state.manifest = await vault.fetchAndDecryptManifest(state.gitHost, handle);
+      // Record sync time (unix SECONDS) for the get_vault_status indicator.
+      state.gitHost.lastSyncAt = Math.floor(Date.now() / 1000);
       return { ok: true };
     }
 
@@ -634,9 +636,9 @@ export async function handle(
     case 'attach_vault':
       return vault.handleAttachVault(msg, state);
 
-    // get_vault_status lands in Phase 6 (Dev-C).
-    // Until each case lands, an unhandled popup message returns an explicit
-    // error rather than falling through with no return.
+    case 'get_vault_status':
+      return vault.handleGetVaultStatus(state);
+
     default:
       return { ok: false, error: `unhandled popup message: ${(msg as { type: string }).type}` };
   }