fix(mcp): normalize CRLF before frontmatter fence detection

Round-4 review found a fifth bypass, and this one needs no adversarial
old_str/new_str crafting at all: frontmatterBlock's fence detection matches
the literal byte sequence "---\n". A note whose fence lines use CRLF
("---\r\n") fails strings.HasPrefix(doc, "---\n") on the precondition read
purely from line-ending encoding — hadFence is false, so the round-3/round-4
checks can never fire, frontmatterScalars falls back to an empty map, and
claim.ValidateEnvelope({}) is nil by design. An invalid claim_provenance
value lands with zero error.

Zero CRLF-fenced notes exist in the live corpus today, but nothing in the
code prevented one, and this class of encoding-dependent false negative
could resurface in any of the other fence-based checks. Normalize once
inside frontmatterBlock so every caller (fence-presence detection and
frontmatterScalars) agrees regardless of line ending.

The broader structural concern — three independent, subtly inconsistent
fence-parsing implementations across internal/mcp, internal/api, and
internal/frontmatter, of which this CRLF gap is a direct symptom — is out
of scope for this PR and filed separately as hyperguild#138.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

ingestion/internal/mcp/tools_notes.go

@@ -310,6 +310,13 @@ func checkClaimEnvelope(brainDir, path, oldStr, newStr string) error {
 // frontmatterBlock returns the raw text between doc's opening and closing
 // `---` fences, or ok=false if doc has no (properly closed) fence.
 func frontmatterBlock(doc string) (block string, ok bool) {
+	// Round 4 found a note with CRLF fence lines ("---\r\n") silently
+	// defeats HasPrefix(doc, "---\n") on the precondition read — not an
+	// adversarial old_str/new_str, just an encoding difference — which
+	// makes checkClaimEnvelope no-op entirely. Normalize once, here, so
+	// every caller (hadFence/hasFence detection and frontmatterScalars)
+	// agrees regardless of line ending.
+	doc = strings.ReplaceAll(doc, "\r\n", "\n")
 	if !strings.HasPrefix(doc, "---\n") {
 		return "", false
 	}

ingestion/internal/mcp/tools_notes_test.go

@@ -196,6 +196,33 @@ func TestBrainEdit_RejectsAnEditThatRemovesTheClosingFence(t *testing.T) {
 	assert.Contains(t, string(got), "claim_provenance: asserted\n---\n", "rejected edit must leave the note byte-identical, fence intact")
 }
 
+func TestBrainEdit_RejectsAnInvalidEnvelopeOnACRLFFencedNote(t *testing.T) {
+	// Found in round-4 review: frontmatterBlock's fence detection matches
+	// the literal byte sequence "---\n" / "\n---\n". A note whose fence
+	// lines use CRLF ("---\r\n") fails strings.HasPrefix(doc, "---\n") on
+	// the BEFORE read, so hadFence is false — not because the fence is
+	// missing, but because \r sits in front of every \n. hadFence&&!hasFence
+	// can then never fire, frontmatterScalars falls back to an empty map,
+	// and claim.ValidateEnvelope({}) is nil by design. No adversarial
+	// old_str/new_str crafting needed: encoding alone silently disables the
+	// whole envelope check for this note. Zero CRLF-fenced notes exist in
+	// the live corpus today, but nothing in the code prevents one.
+	dir := newBrainRepo(t)
+	note := filepath.Join(dir, "wiki/homelab/facts/n.md")
+	require.NoError(t, os.WriteFile(note,
+		[]byte("---\r\ntitle: 'n'\r\nhalf_life: durable\r\napplies: universal\r\nclaim_provenance: asserted\r\n---\r\n\r\nbody\r\n"), 0o644))
+	srv := mcp.NewServer(dir, nil, nil, nil)
+
+	resp := call(t, srv, "brain_edit", map[string]any{
+		"path": "wiki/homelab/facts/n.md", "old_str": "asserted", "new_str": "guessed",
+	})
+	require.Contains(t, resp, "error", "%v", resp)
+
+	got, err := os.ReadFile(note)
+	require.NoError(t, err)
+	assert.Contains(t, string(got), "claim_provenance: asserted", "rejected edit must leave the note byte-identical")
+}
+
 func TestBrainEdit_AllowsAValidClaimEnvelopeEdit(t *testing.T) {
 	dir := newBrainRepo(t)
 	note := filepath.Join(dir, "wiki/homelab/facts/n.md")