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/brainfs/brainfs.go

@@ -133,7 +133,8 @@ func (b *Brain) readOpenedMax(rel string, max int64) ([]byte, error) {
 }
 
 // wrapAfterStat is a test seam: it runs between the size check and the read,
-// and wraps the reader the read consumes.
+// and wraps the reader the read consumes. A package global: brainfs tests
+// must not use t.Parallel.
 var wrapAfterStat func(io.Reader) io.Reader
 
 // WalkRegular walks a brain-relative directory through the root and calls fn
@@ -141,7 +142,21 @@ var wrapAfterStat func(io.Reader) io.Reader
 // devices are skipped, never followed). An entry error is passed to onErr and
 // the walk continues; a nil onErr ignores it.
 func (b *Brain) WalkRegular(relDir string, fn func(rel string) error, onErr func(rel string, err error)) error {
-	return fs.WalkDir(b.root.FS(), filepath.ToSlash(relDir), func(p string, d fs.DirEntry, err error) error {
+	relDir = filepath.ToSlash(relDir)
+	// fs.WalkDir stats its root with a follow, and os.Root allows a symlink
+	// that stays inside the brain: knowledge -> . would alias the whole
+	// brain. Require a real directory first.
+	fi, err := b.root.Lstat(relDir)
+	if err == nil && !fi.IsDir() {
+		err = fmt.Errorf("%s: walk root is not a directory: %w", relDir, ErrNotRegular)
+	}
+	if err != nil {
+		if onErr != nil {
+			onErr(relDir, err)
+		}
+		return nil
+	}
+	return fs.WalkDir(b.root.FS(), relDir, func(p string, d fs.DirEntry, err error) error {
 		if err != nil {
 			if onErr != nil {
 				onErr(p, err)