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

BuildWingIndex/BuildAllWingIndexes listed wiki/<wing>/<hall>/ with
os.ReadDir and opened each note by path: a symlinked note put the
target's title into _index.md (110k leaks in the swap race), a symlinked
wing was followed and its _index.md written into the target, and a FIFO
hung the build (it runs on every brain_write).

- New brainfs.ReadDirDirs: real subdirectories only, every component of
  the dir a real directory. Wings and halls are listed with it and
  ReadDirRegular; titles are read with ReadRegular
  (readTitleAndCreatedIn). A symlinked wing is an error; a symlinked hall
  is skipped.
- linkcheck entryIndex walks with WalkRegular: a symlinked .md name no
  longer makes a [[link]] resolve.
- readTitleAndCreated (path-based) stays for tunnel.DetectTunnels until
  PR #170 lands; that is slice 5b.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.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) {