fix(graph,wiki): read the brain through brainfs (#157, slice 4)

graphsync.IndexDoc/BackfillFromBrainDir and wiki.LoadInventory/
RebuildIndex read notes by path (os.ReadFile, os.Open, os.ReadDir +
open). A symlinked note put the target's title, aliases and wikilinks
into the knowledge graph, the wiki inventory (and the LLM prompts built
from it) and wiki/index.md; a FIFO hung each read.

- graphsync: IndexDoc and the backfill read through brainfs; the
  backfill walks with WalkRegular, skips a missing root and aborts on
  any other walk error, including a symlinked root.
- wiki: LoadInventory lists type dirs with the new
  brainfs.ReadDirRegular (every component a real dir, regular files
  only) and reads frontmatter through the root; a symlinked type dir is
  an error. RebuildIndex reads first sentences through the root.
- brainfstest's outside file now carries the sentinel in frontmatter
  title and aliases, a heading and a wikilink, so extractors that ignore
  the body are tested too.

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

ingestion/internal/brainfs/brainfs.go

@@ -82,6 +82,28 @@ func (b *Brain) realDir(rel string) error {
 	return nil
 }
 
+// ReadDirRegular lists the regular files directly in a brain-relative
+// directory, by name. Every component of relDir must be a real directory
+// (ErrNotRegular otherwise); symlinks, FIFOs, devices and subdirectories in
+// it are left out.
+func (b *Brain) ReadDirRegular(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().IsRegular() {
+			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) {

ingestion/internal/brainfs/brainfstest/brainfstest.go

@@ -20,11 +20,16 @@ import (
 // Sentinel marks the outside file's contents.
 const Sentinel = "zqbrainfssentinelonefiftyseven"
 
+// outsideContent carries Sentinel in every field a reader might extract: the
+// frontmatter title and aliases, a heading, a wikilink and the body.
+const outsideContent = "---\ntitle: " + Sentinel + "\naliases:\n  - " + Sentinel + "\n---\n# " + Sentinel +
+	"\n\n" + Sentinel + " [[" + Sentinel + "]]\n"
+
 // Outside writes a file outside any brain dir containing Sentinel.
 func Outside(t *testing.T) string {
 	t.Helper()
 	p := filepath.Join(t.TempDir(), "secret.md")
-	if err := os.WriteFile(p, []byte("# secret\n\n"+Sentinel), 0o644); err != nil {
+	if err := os.WriteFile(p, []byte(outsideContent), 0o644); err != nil {
 		t.Fatal(err)
 	}
 	return p