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/__tests__/vault-status.test.ts

@@ -0,0 +1,53 @@
+import { describe, expect, it } from 'vitest';
+import { handleGetVaultStatus } from '../vault';
+import type { Manifest, ManifestEntry } from '../../shared/types';
+
+// The handler only reads gitHost's three cache fields, so the test feeds a
+// minimal object — the handler's Pick-typed param makes full GitHost mocking
+// unnecessary.
+const cache = (lastSyncAt: number | null, ahead = 0, behind = 0) =>
+  ({ lastSyncAt, ahead, behind });
+
+function manifestWith(activeCount: number, trashedCount = 0): Manifest {
+  const items: Record<string, ManifestEntry> = {};
+  for (let i = 0; i < activeCount; i++) {
+    items[`a${i}`] = { trashed_at: undefined } as ManifestEntry;
+  }
+  for (let i = 0; i < trashedCount; i++) {
+    items[`t${i}`] = { trashed_at: 1000 } as ManifestEntry;
+  }
+  return { items } as Manifest;
+}
+
+describe('handleGetVaultStatus', () => {
+  it('returns zeros when never synced and no manifest', () => {
+    const resp = handleGetVaultStatus({ gitHost: cache(null), manifest: null });
+    expect(resp).toEqual({
+      ok: true,
+      data: { ahead: 0, behind: 0, lastSyncAt: null, pendingItems: 0 },
+    });
+  });
+
+  it('reflects cached sync state + active (non-trashed) item count', () => {
+    const resp = handleGetVaultStatus({
+      gitHost: cache(1234567890, 3, 1),
+      manifest: manifestWith(5, 2),
+    });
+    expect(resp.ok).toBe(true);
+    if (resp.ok) {
+      expect(resp.data).toEqual({
+        ahead: 3, behind: 1, lastSyncAt: 1234567890, pendingItems: 5,
+      });
+    }
+  });
+
+  it('returns vault_locked error when gitHost is null', () => {
+    expect(handleGetVaultStatus({ gitHost: null, manifest: null }))
+      .toEqual({ ok: false, error: 'vault_locked' });
+  });
+
+  it('is synchronous — no network round-trip', () => {
+    const resp = handleGetVaultStatus({ gitHost: cache(0), manifest: null });
+    expect(resp).not.toBeInstanceOf(Promise);
+  });
+});