fix(tunnel): DetectTunnels walks and reads through brainfs (#174, #157 slice 5b)

DetectTunnels runs inside every brain_write and brain_update
(wikiUpkeep -> AutoTunnel). It walked wiki/ with filepath.WalkDir and
opened each note with a blocking os.Open: a FIFO named *.md anywhere
under wiki/ hung the write with no deadline, and a symlinked note's
title became a tunnel candidate (171k leaks in the swap race).

It now walks with brainfs.WalkRegular (regular files only) and reads
titles with readTitleAndCreatedIn (ReadRegular: O_NONBLOCK, handle and
SameFile checks). A wiki/ that is not a real directory yields no
candidates, as filepath.WalkDir did; other walk errors still abort. The
path-based readTitleAndCreated, now unused, is removed.

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

ingestion/internal/brain/index.go

@@ -116,22 +116,6 @@ func collectWingEntries(bfs *brainfs.Brain, wingRel string) ([]noteEntry, error)
 	return out, nil
 }
 
-// readTitleAndCreated reads YAML frontmatter for title + created_at; falls
-// back to slug and file mtime when absent.
-func readTitleAndCreated(path, slug string) (string, string) {
-	f, err := os.Open(path)
-	if err != nil {
-		return slug, ""
-	}
-	defer func() { _ = f.Close() }()
-	title, created := parseTitleAndCreated(f)
-	if created == "" {
-		if info, err := os.Stat(path); err == nil {
-			created = info.ModTime().UTC().Format("2006-01-02")
-		}
-	}
-	return titleOrSlug(title, slug), created
-}
 
 // readTitleAndCreatedIn is readTitleAndCreated through the brain root,
 // regular files only (hyperguild#157).