fix(brain): wing indexes and the link check read through brainfs (#157, slice 5a) (#175)

BuildWingIndex/BuildAllWingIndexes list wings and halls with the new brainfs.ReadDirDirs and ReadDirRegular and read titles with ReadRegular: a symlinked note no longer reaches _index.md, a symlinked wing is refused (nothing written into its target), a symlinked hall is skipped. linkcheck entryIndex walks with WalkRegular, so a symlinked .md cannot make a link resolve, and a linked tree that is not a real directory is skipped with a warning (brain_write still lands).

Refs #157. Adversarial review: BLOCKED, PASS. Follow-up: #178.

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

ingestion/internal/brainfs/brainfs.go

@@ -104,6 +104,27 @@ func (b *Brain) ReadDirRegular(relDir string) ([]string, error) {
 	return names, nil
 }
 
+// ReadDirDirs lists the real subdirectories directly in a brain-relative
+// directory, by name. Every component of relDir must be a real directory
+// (ErrNotRegular otherwise); symlinks to directories are left out.
+func (b *Brain) ReadDirDirs(relDir string) ([]string, error) {
+	relDir = filepath.ToSlash(relDir)
+	if err := b.realDir(relDir); err != nil {
+		return nil, err
+	}
+	ents, err := fs.ReadDir(b.root.FS(), relDir)
+	if err != nil {
+		return nil, err
+	}
+	var names []string
+	for _, e := range ents {
+		if e.Type().IsDir() {
+			names = append(names, e.Name())
+		}
+	}
+	return names, nil
+}
+
 // ReadRegular reads a brain-relative path, refusing anything that is not a
 // regular file, both before the open and on the opened handle.
 func (b *Brain) ReadRegular(rel string) ([]byte, error) {