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/sync_test.go

@@ -7,6 +7,7 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/mathiasbq/hyperguild/ingestion/internal/vectorstore"
 	"github.com/stretchr/testify/assert"
@@ -14,16 +15,27 @@ import (
 )
 
 type stubStore struct {
-	known    map[string]struct{}
+	// known maps chunk-path → updated_at. Tests that don't care about
+	// re-embed-on-mtime use a far-future time so the Sync skip path
+	// always wins. Tests that do exercise the mtime path set the
+	// updated_at explicitly.
+	known    map[string]time.Time
 	upserts  map[string][]float32
 	deletes  []string
 	failNext error
 }
 
-func (s *stubStore) KnownPaths(_ context.Context) (map[string]struct{}, error) {
-	out := make(map[string]struct{}, len(s.known))
-	for k := range s.known {
-		out[k] = struct{}{}
+// farFuture is "newer than any file mtime", used as the default
+// updated_at in stubs that don't care about re-embed behavior.
+var farFuture = time.Now().Add(24 * time.Hour)
+
+func (s *stubStore) KnownPathsWithTime(_ context.Context) (map[string]time.Time, error) {
+	out := make(map[string]time.Time, len(s.known))
+	for k, t := range s.known {
+		if t.IsZero() {
+			t = farFuture
+		}
+		out[k] = t
 	}
 	return out, nil
 }
@@ -67,7 +79,7 @@ func TestSync_AddsNewFiles(t *testing.T) {
 	writeNote(t, dir, "wiki/jepa-fx/facts/x.md", "body of x")
 	writeNote(t, dir, "wiki/jepa-fx/facts/y.md", "body of y")
 
-	store := &stubStore{known: map[string]struct{}{}}
+	store := &stubStore{known: map[string]time.Time{}}
 	emb := stubEmbedder{vec: make([]float32, 768)}
 	res, err := vectorstore.Sync(context.Background(), dir, store, emb)
 	require.NoError(t, err)
@@ -81,7 +93,7 @@ func TestSync_SkipsAlreadyKnown(t *testing.T) {
 	dir := t.TempDir()
 	writeNote(t, dir, "wiki/a/facts/x.md", "x")
 
-	store := &stubStore{known: map[string]struct{}{"wiki/a/facts/x.md#0001": {}}}
+	store := &stubStore{known: map[string]time.Time{"wiki/a/facts/x.md#0001": {}}}
 	emb := stubEmbedder{vec: make([]float32, 768)}
 	res, err := vectorstore.Sync(context.Background(), dir, store, emb)
 	require.NoError(t, err)
@@ -93,7 +105,7 @@ func TestSync_DeletesDisappearedFiles(t *testing.T) {
 	dir := t.TempDir()
 	require.NoError(t, os.MkdirAll(filepath.Join(dir, "wiki"), 0o755))
 	// store has a path that doesn't exist on disk anymore
-	store := &stubStore{known: map[string]struct{}{"wiki/old/facts/ghost.md#0001": {}}}
+	store := &stubStore{known: map[string]time.Time{"wiki/old/facts/ghost.md#0001": {}}}
 	res, err := vectorstore.Sync(context.Background(), dir, &stubStoreWithDelete{stubStore: store}, stubEmbedder{vec: make([]float32, 768)})
 	require.NoError(t, err)
 	assert.Equal(t, 1, res.Deleted)
@@ -111,7 +123,7 @@ func TestSync_SkipsIndexFiles(t *testing.T) {
 	writeNote(t, dir, "wiki/a/_index.md", "moc")
 	writeNote(t, dir, "wiki/a/facts/real.md", "body")
 
-	store := &stubStore{known: map[string]struct{}{}}
+	store := &stubStore{known: map[string]time.Time{}}
 	res, err := vectorstore.Sync(context.Background(), dir, store, stubEmbedder{vec: make([]float32, 768)})
 	require.NoError(t, err)
 	assert.Equal(t, 1, res.Added)
@@ -123,7 +135,7 @@ func TestSync_ScansKnowledgeDir(t *testing.T) {
 	writeNote(t, dir, "wiki/a/facts/x.md", "x")
 	writeNote(t, dir, "knowledge/2026-05-19-koala-gpu-setup.md", "knowledge body")
 
-	store := &stubStore{known: map[string]struct{}{}}
+	store := &stubStore{known: map[string]time.Time{}}
 	emb := stubEmbedder{vec: make([]float32, 768)}
 	res, err := vectorstore.Sync(context.Background(), dir, store, emb)
 	require.NoError(t, err)
@@ -143,7 +155,7 @@ func TestSync_ChunksLongFiles(t *testing.T) {
 	}
 	writeNote(t, dir, "knowledge/long.md", body)
 
-	store := &stubStore{known: map[string]struct{}{}}
+	store := &stubStore{known: map[string]time.Time{}}
 	emb := stubEmbedder{vec: make([]float32, 768)}
 	res, err := vectorstore.Sync(context.Background(), dir, store, emb)
 	require.NoError(t, err)
@@ -164,7 +176,7 @@ func TestSync_ShortFileGetsSingleChunkRow(t *testing.T) {
 	dir := t.TempDir()
 	writeNote(t, dir, "wiki/short.md", "tiny body\n")
 
-	store := &stubStore{known: map[string]struct{}{}}
+	store := &stubStore{known: map[string]time.Time{}}
 	emb := stubEmbedder{vec: make([]float32, 768)}
 	res, err := vectorstore.Sync(context.Background(), dir, store, emb)
 	require.NoError(t, err)
@@ -176,7 +188,7 @@ func TestSync_SkipsFileIfAnyChunkAlreadyKnown(t *testing.T) {
 	dir := t.TempDir()
 	writeNote(t, dir, "wiki/foo.md", "body\n")
 
-	store := &stubStore{known: map[string]struct{}{
+	store := &stubStore{known: map[string]time.Time{
 		"wiki/foo.md#0001": {},
 	}}
 	emb := stubEmbedder{vec: make([]float32, 768)}
@@ -189,7 +201,7 @@ func TestSync_SkipsFileIfAnyChunkAlreadyKnown(t *testing.T) {
 func TestSync_DeletesAllChunksOfDisappearedFile(t *testing.T) {
 	dir := t.TempDir()
 	require.NoError(t, os.MkdirAll(filepath.Join(dir, "wiki"), 0o755))
-	store := &stubStore{known: map[string]struct{}{
+	store := &stubStore{known: map[string]time.Time{
 		"wiki/ghost.md#0001": {},
 		"wiki/ghost.md#0002": {},
 		"wiki/ghost.md#0003": {},
@@ -199,6 +211,49 @@ func TestSync_DeletesAllChunksOfDisappearedFile(t *testing.T) {
 	assert.Equal(t, 3, res.Deleted)
 }
 
+func TestSync_ReembedsFileWhenMtimeNewer(t *testing.T) {
+	dir := t.TempDir()
+	writeNote(t, dir, "wiki/edited.md", "original body\n")
+	// Force the file's mtime ahead of any plausible store updated_at.
+	future := time.Now().Add(1 * time.Hour)
+	require.NoError(t, os.Chtimes(filepath.Join(dir, "wiki/edited.md"), future, future))
+
+	store := &stubStore{
+		known: map[string]time.Time{
+			// Existing chunk row pre-dates the file's mtime.
+			"wiki/edited.md#0001": time.Now().Add(-1 * time.Hour),
+		},
+	}
+	emb := stubEmbedder{vec: make([]float32, 768)}
+	res, err := vectorstore.Sync(context.Background(), dir, store, emb)
+	require.NoError(t, err)
+	assert.Equal(t, 1, res.Added, "file with newer mtime should be re-embedded")
+	assert.Contains(t, store.upserts, "wiki/edited.md#0001")
+	// Old chunks of the same parent must be deleted before re-embed so
+	// shrunk files don't leave orphan rows at higher #NNNN indexes.
+	assert.Contains(t, store.deletes, "wiki/edited.md#0001")
+}
+
+func TestSync_SkipsFileWhenMtimeOlder(t *testing.T) {
+	dir := t.TempDir()
+	writeNote(t, dir, "wiki/stable.md", "body\n")
+	// Backdate mtime to before the store's recorded updated_at.
+	past := time.Now().Add(-2 * time.Hour)
+	require.NoError(t, os.Chtimes(filepath.Join(dir, "wiki/stable.md"), past, past))
+
+	store := &stubStore{
+		known: map[string]time.Time{
+			"wiki/stable.md#0001": time.Now(),
+		},
+	}
+	emb := stubEmbedder{vec: make([]float32, 768)}
+	res, err := vectorstore.Sync(context.Background(), dir, store, emb)
+	require.NoError(t, err)
+	assert.Equal(t, 0, res.Added)
+	assert.Empty(t, store.upserts)
+	assert.Empty(t, store.deletes)
+}
+
 func TestSync_NoOpWhenComponentsNil(t *testing.T) {
 	dir := t.TempDir()
 	writeNote(t, dir, "wiki/a/facts/x.md", "x")
@@ -210,7 +265,7 @@ func TestSync_NoOpWhenComponentsNil(t *testing.T) {
 func TestSync_CollectsEmbedderErrors(t *testing.T) {
 	dir := t.TempDir()
 	writeNote(t, dir, "wiki/a/facts/x.md", "x")
-	store := &stubStore{known: map[string]struct{}{}}
+	store := &stubStore{known: map[string]time.Time{}}
 	emb := stubEmbedder{err: errors.New("upstream down")}
 	res, err := vectorstore.Sync(context.Background(), dir, store, emb)
 	require.NoError(t, err)