fix(brainfs): refuse a symlinked walk root (#157)

Review round 1 on PR #166: fs.WalkDir stats its root with a follow, and
os.Root allows a symlink that stays inside the brain, so knowledge -> .
aliased raw/, sessions/ and root files into the embeddings (and a
wiki/<w> -> .. root into search results). On main, filepath.WalkDir
Lstat'ed the root and did not descend.

- WalkRegular Lstats its root and requires a real directory; anything
  else goes to onErr as ErrNotRegular and nothing is yielded.
- Sync skips a missing scan root via bfs.Lstat before walking, and every
  other walk error, including a symlinked root, aborts before the delete
  pass. This removes the unpinned rel == sub special case.
- Search warns and skips a symlinked root (new test).
- Test renamed to TestSyncRefusesSymlinkedNotesAndSurvivesSwapRaces.
- wrapAfterStat comment: brainfs tests must not use t.Parallel.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ingestion/internal/vectorstore/sync.go

@@ -103,11 +103,13 @@ func Sync(ctx context.Context, brainDir string, store Store, embedder Embedder)
 	defer func() { _ = bfs.Close() }()
 
 	for _, sub := range scanDirs {
+		if _, err := bfs.Lstat(sub); errors.Is(err, fs.ErrNotExist) {
+			continue
+		}
+		// Any other walk error, including a scan root that is a symlink,
+		// aborts the run below.
 		var walkErr error
 		onErr := func(rel string, err error) {
-			if rel == sub && errors.Is(err, fs.ErrNotExist) {
-				return
-			}
 			if walkErr == nil {
 				walkErr = fmt.Errorf("%s: %w", rel, err)
 			}