fix(ingestion): an unlistable brain root is a scan error, not clean

PR #159 round 2: with the brain root at mode 0100 (enterable, not
listable) WalkDir failed at the root, Scan discarded the error and
reported clean, and startup passed while search, which goes straight
into wiki/, served a planted symlink. Scan now returns the root's walk
error, so startup refuses on anything but a missing root. Also pins
that a cleared set is reported again when it reappears.

Refs #157

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

ingestion/internal/symlinkguard/symlinkguard.go

@@ -62,8 +62,9 @@ func (r Result) String() string {
 // directory it could not read, continuing past both. Only the ROOT .git is
 // skipped (it is brain-sync's, and no reader walks it); a nested .git is
 // scanned like any other directory. brainDir itself may be a symlink (a
-// volume mount); it is resolved first. The error is non-nil only when the
-// root cannot be resolved, wrapping os.ErrNotExist when it is missing.
+// volume mount); it is resolved first. The error is non-nil when the root
+// cannot be resolved (wrapping os.ErrNotExist when it is missing) or cannot be
+// listed.
 func Scan(brainDir string) (Result, error) {
 	root, err := filepath.EvalSymlinks(brainDir)
 	if err != nil {
@@ -71,7 +72,7 @@ func Scan(brainDir string) (Result, error) {
 	}
 	var r Result
 	rootGit := filepath.Join(root, ".git")
-	_ = filepath.WalkDir(root, func(p string, d fs.DirEntry, err error) error {
+	walkErr := filepath.WalkDir(root, func(p string, d fs.DirEntry, err error) error {
 		rel, _ := filepath.Rel(root, p)
 		rel = filepath.ToSlash(rel)
 		if err != nil {
@@ -94,6 +95,13 @@ func Scan(brainDir string) (Result, error) {
 	})
 	sort.Strings(r.Symlinks)
 	sort.Strings(r.Unreadable)
+	if walkErr != nil {
+		// Only the root's own error reaches here (deeper errors are recorded
+		// as Unreadable). A root that cannot be listed cannot be shown clean
+		// (PR #159 round 2: discarding this reported clean while search
+		// served a planted symlink).
+		return r, fmt.Errorf("scan brain dir: %w", walkErr)
+	}
 	return r, nil
 }