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/search/brainroot_test.go

@@ -61,6 +61,19 @@ func TestQueryDoesNotFollowASymlinkedDirectory(t *testing.T) {
 	assert.Zero(t, leaked(rs))
 }
 
+// A search root that is a symlink staying inside the brain (wiki/<w> -> ..)
+// was followed by fs.WalkDir, aliasing raw/ and root files into results
+// (PR #166 review). The root is now Lstat-checked and skipped.
+func TestQueryDoesNotFollowASymlinkedSearchRootInsideTheBrain(t *testing.T) {
+	brain, _, _ := searchBrain(t)
+	require.NoError(t, os.MkdirAll(filepath.Join(brain, "raw"), 0o755))
+	require.NoError(t, os.WriteFile(filepath.Join(brain, "raw", "r.md"), []byte("# r\n\nzqrawonly"), 0o644))
+	require.NoError(t, os.Symlink("..", filepath.Join(brain, "wiki", "alias")))
+	rs, err := Query(brain, QueryOptions{Query: "zqrawonly", Wing: "alias", Limit: 10})
+	require.NoError(t, err)
+	assert.Empty(t, rs)
+}
+
 // swapInto keeps renaming a regular note, and either a symlink to outside or
 // a FIFO, into path. Each FIFO has a writer held open (as any local process
 // can): a FIFO with no writer reads EOF at once, which would let a reader