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_root_test.go

@@ -38,7 +38,7 @@ func (e *recordingEmbedder) all() string {
 // hyperguild#157 slice 3: Sync walked wiki/ and knowledge/ and read each .md
 // with os.ReadFile, so a symlinked note had its target embedded and served by
 // semantic search, and a FIFO stalled the sync.
-func TestSyncRefusesSymlinksAndSurvivesSwapRaces(t *testing.T) {
+func TestSyncRefusesSymlinkedNotesAndSurvivesSwapRaces(t *testing.T) {
 	brain := t.TempDir()
 	writeNote(t, brain, "wiki/w/facts/ok.md", "inside note")
 	require.NoError(t, os.Symlink(brainfstest.Outside(t), filepath.Join(brain, "wiki", "w", "facts", "leak.md")))
@@ -95,3 +95,32 @@ func TestSyncAbortsBeforeDeletingWhenASubdirIsUnreadable(t *testing.T) {
 	assert.Error(t, err)
 	assert.Empty(t, store.deletes)
 }
+
+// A scan root that is a symlink inside the brain (knowledge -> .) was
+// followed by fs.WalkDir, aliasing raw/, sessions/ and root files into the
+// index. Sync now aborts on it, before embedding or deleting anything.
+func TestSyncAbortsOnASymlinkedScanRoot(t *testing.T) {
+	brain := t.TempDir()
+	writeNote(t, brain, "wiki/w/a.md", "wiki note")
+	writeNote(t, brain, "sessions/s1.md", "session note")
+	require.NoError(t, os.Symlink(".", filepath.Join(brain, "knowledge")))
+	emb := &recordingEmbedder{}
+	store := &stubStore{known: map[string]time.Time{"wiki/w/a.md#0001": {}}}
+	_, err := vectorstore.Sync(context.Background(), brain, store, emb)
+	assert.Error(t, err)
+	assert.NotContains(t, emb.all(), "session note")
+	for p := range store.upserts {
+		assert.False(t, strings.HasPrefix(p, "knowledge/"), "upserted %s", p)
+	}
+	assert.Empty(t, store.deletes)
+}
+
+// A missing scan root is skipped; the other root still syncs.
+func TestSyncSkipsAMissingScanRoot(t *testing.T) {
+	brain := t.TempDir()
+	writeNote(t, brain, "wiki/w/a.md", "wiki note")
+	emb := &recordingEmbedder{}
+	_, err := vectorstore.Sync(context.Background(), brain, &stubStore{}, emb)
+	require.NoError(t, err)
+	assert.Contains(t, emb.all(), "wiki note")
+}