fix(brain): close round-3 review findings on provenance

- B1: search.extractProvenance locates the frontmatter with
  frontmatter.Split (fence at column 0), so an indented "---" inside a
  block scalar no longer hides provenance from brain_query.
- B2: brain_edit refuses to edit without a snapshot, and if the stamp
  cannot be verified restores the note and reports failure; a failed
  restore is reported as exactly that, never as "rolled back".
- B3: internal/provenance gets direct tests for the whole-document
  backstop (a stamp that cannot read back is refused), standalone group
  decoding, duplicates, lone CR, tab continuations and unclosable
  fences. Removing verify or the restore now fails the suite.
- Principals YAML would read as non-strings (true, 123, null, ~) are
  quoted instead of refused. An unclosable fence ("---x") is refused
  rather than demoted into the body. writeHallNote creates the hall
  directory only after the note passes, so a refused write leaves
  nothing behind (brain#21).
- MCP brain_ingest (stub LLM) asserted to stamp the caller.

Refs #145 #144

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

ingestion/internal/mcp/tools_notes.go

@@ -110,10 +110,17 @@ func (s *Server) brainEdit(ctx context.Context, args json.RawMessage) (json.RawM
 	// verified, the edit is rolled back rather than left on disk unattributed
 	// (or carrying whatever provenance the edit wrote).
 	var a brainEditArgs
-	_ = json.Unmarshal(args, &a)
-	var prior []byte
-	if t, rerr := notes.Resolve(s.brainDir, a.Path, notes.ScopeNotes); rerr == nil {
-		prior, _ = os.ReadFile(t.Abs)
+	if err := json.Unmarshal(args, &a); err != nil {
+		return nil, fmt.Errorf("parse args: %w", err)
+	}
+	target, err := notes.Resolve(s.brainDir, a.Path, notes.ScopeNotes)
+	if err != nil {
+		return nil, err
+	}
+	// No snapshot, no edit: without one a failed stamp could not be undone.
+	prior, err := os.ReadFile(target.Abs)
+	if err != nil {
+		return nil, fmt.Errorf("snapshot before edit: %w", err)
 	}
 	res, out, err := s.editWithScope(args, notes.ScopeNotes)
 	if err != nil {
@@ -124,8 +131,10 @@ func (s *Server) brainEdit(ctx context.Context, args json.RawMessage) (json.RawM
 	// to the editor after every edit (PR #147 review, escape E1).
 	origin, writtenBy := provenanceFrom(ctx)
 	if serr := api.StampProvenanceFile(s.brainDir, res.Path, origin, writtenBy); serr != nil {
-		if t, rerr := notes.Resolve(s.brainDir, res.Path, notes.ScopeNotes); rerr == nil && prior != nil {
-			_ = os.WriteFile(t.Abs, prior, 0o644)
+		if werr := os.WriteFile(target.Abs, prior, 0o644); werr != nil {
+			// Say exactly what state the note is in: edited, unverified.
+			return nil, fmt.Errorf("stamp provenance failed (%v) AND the edit could NOT be rolled back: %w; "+
+				"%s carries the edit under its previous attribution", serr, werr, res.Path)
 		}
 		return nil, fmt.Errorf("edit rolled back, stamp provenance: %w", serr)
 	}