feat(vectorstore): re-embed on file mtime > store updated_at (#23)

Removes the TODO in Sync that left files static after their first embed.
Edits to brain/wiki/ and brain/knowledge/ now surface in subsequent
syncs without manual /backfill-embeddings calls.

Approach
- Store interface: KnownPaths → KnownPathsWithTime returning path →
  updated_at. Callers compare against file mtime to detect edits.
- PGStore: SELECT path, updated_at FROM brain_embeddings.
- Sync groups known chunks by parent path and tracks the EARLIEST
  updated_at per parent. A file is stale when its mtime is after that
  oldest chunk's timestamp — any chunk older than the file means at
  least one chunk hasn't been refreshed since the last edit.
- Stale-path rewrite: delete every old chunk for the parent (handles
  "file shrunk → fewer chunks → orphan rows at higher #NNNN" cleanly),
  then re-chunk + re-embed + re-upsert.

Tests
- New: TestSync_ReembedsFileWhenMtimeNewer — file mtime forced into the
  future vs store updated_at; Sync deletes old chunk + upserts fresh one.
- New: TestSync_SkipsFileWhenMtimeOlder — file mtime backdated; Sync is
  a no-op (no upserts, no deletes).
- Updated: stubStore.known is now map[string]time.Time. A zero value
  resolves to a far-future sentinel so existing "skip if already known"
  tests keep passing without per-test setup.
- pg_test renamed KnownPaths integration → KnownPathsWithTime; asserts
  updated_at is non-zero and within 5s of insert wall-clock.

Backward compat
- brain_embeddings rows pre-dating this change carry valid updated_at
  values (column was always populated via `DEFAULT now()` + ON CONFLICT
  `updated_at = now()`). No migration needed. Live pod will start
  re-embedding any file whose source has been edited since its chunks
  were originally written.

Closes gitea/mathias/hyperguild#23.

ingestion/internal/vectorstore/pg_test.go

@@ -36,7 +36,7 @@ func freshStore(t *testing.T) (*vectorstore.PGStore, context.Context) {
 	t.Cleanup(s.Close)
 	require.NoError(t, s.Init(ctx))
 	// Clean slate per test.
-	_, _ = s.KnownPaths(ctx)
+	_, _ = s.KnownPathsWithTime(ctx)
 	require.NoError(t, s.Delete(ctx, "%test-fixture%"))
 	return s, ctx
 }
@@ -67,15 +67,18 @@ func TestIntegration_UpsertAndSearch(t *testing.T) {
 	})
 }
 
-func TestIntegration_KnownPaths(t *testing.T) {
+func TestIntegration_KnownPathsWithTime(t *testing.T) {
 	s, ctx := freshStore(t)
+	before := time.Now()
 	require.NoError(t, s.Upsert(ctx, "wiki/k.md", vec(768, 0.5)))
 	t.Cleanup(func() { _ = s.Delete(ctx, "wiki/k.md") })
 
-	paths, err := s.KnownPaths(ctx)
+	paths, err := s.KnownPathsWithTime(ctx)
 	require.NoError(t, err)
-	_, ok := paths["wiki/k.md"]
-	assert.True(t, ok)
+	at, ok := paths["wiki/k.md"]
+	require.True(t, ok)
+	assert.False(t, at.IsZero(), "updated_at must not be zero")
+	assert.WithinDuration(t, before, at, 5*time.Second, "updated_at must be recent")
 }
 
 func TestUpsert_RejectsWrongDimension(t *testing.T) {