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/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