fix(vectorstore): walk and read the brain through brainfs (#157, slice 3) (#166)

vectorstore.Sync walks and reads via brainfs (regular files only, handle-checked), so symlinked notes are no longer embedded and their old chunks are purged. brainfs.WalkRegular requires every component of its root to be a real directory (fixes in-brain root aliasing in Sync and search Wing/Wing+Hall). Walk errors, including a symlinked scan root, abort Sync before the delete pass. brainfs gains Lstat. Directory-swap race recorded as accepted residual (#167).

Refs #157. Adversarial review: BLOCKED, BLOCKED, PASS.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Mathias <mthbqv@gmail.com>

ingestion/internal/search/brainroot_test.go

@@ -61,6 +61,31 @@ 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 TestQueryDoesNotFollowASymlinkedWingRootInsideTheBrain(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)
+}
+
+// With a hall the search root is wiki/<wing>/<hall>; a symlinked <wing> with
+// a real <hall> directory under its target was followed (PR #166 round 2).
+func TestQueryDoesNotFollowASymlinkedWingUnderAWingAndHallRoot(t *testing.T) {
+	brain, _, _ := searchBrain(t)
+	require.NoError(t, os.MkdirAll(filepath.Join(brain, "raw", "facts"), 0o755))
+	require.NoError(t, os.WriteFile(filepath.Join(brain, "raw", "facts", "r.md"), []byte("# r\n\nzqrawhall"), 0o644))
+	require.NoError(t, os.Symlink(filepath.Join("..", "raw"), filepath.Join(brain, "wiki", "alias")))
+	rs, err := Query(brain, QueryOptions{Query: "zqrawhall", Wing: "alias", Hall: "facts", 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