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.go

@@ -8,6 +8,7 @@ import (
 	"errors"
 	"fmt"
 	"strings"
+	"time"
 
 	"github.com/jackc/pgx/v5"
 	"github.com/jackc/pgx/v5/pgxpool"
@@ -120,21 +121,26 @@ func (s *PGStore) Search(ctx context.Context, query []float32, limit int) ([]Hit
 	return hits, nil
 }
 
-// KnownPaths returns the path set already present in the store. Used by
-// the watcher to diff against the wiki/ tree and decide what to upsert.
-func (s *PGStore) KnownPaths(ctx context.Context) (map[string]struct{}, error) {
-	rows, err := s.pool.Query(ctx, `SELECT path FROM brain_embeddings`)
+// KnownPathsWithTime returns every embedded chunk path paired with the
+// row's updated_at. Sync uses the timestamps to decide whether a file
+// has been edited since its chunks were last embedded — when the file's
+// mtime exceeds the oldest chunk's updated_at, the file is re-embedded.
+func (s *PGStore) KnownPathsWithTime(ctx context.Context) (map[string]time.Time, error) {
+	rows, err := s.pool.Query(ctx, `SELECT path, updated_at FROM brain_embeddings`)
 	if err != nil {
 		return nil, fmt.Errorf("query paths: %w", err)
 	}
 	defer rows.Close()
-	out := make(map[string]struct{})
+	out := make(map[string]time.Time)
 	for rows.Next() {
-		var p string
-		if err := rows.Scan(&p); err != nil {
+		var (
+			p string
+			t time.Time
+		)
+		if err := rows.Scan(&p, &t); err != nil {
 			return nil, err
 		}
-		out[p] = struct{}{}
+		out[p] = t
 	}
 	return out, rows.Err()
 }