fix(mcp): drop the containment gate, always validate the post-edit envelope

Round-2 review found a third bypass: checkClaimEnvelope required old_str to
be fully contained within the extracted frontmatter block (which excludes
both "---" fence lines). An old_str disambiguated across the closing fence
into the body — a natural choice, since notes.Edit requires old_str to be
unique — failed that containment test, so the gate skipped validation
entirely while notes.Edit applied the replace verbatim, reaching the same
invalid-enum outcome as the two bypasses already closed.

Remove the containment gate structurally instead of patching another edge:
checkClaimEnvelope now always computes the post-edit doc and validates its
resulting frontmatter, regardless of old_str's textual relationship to the
fence. A body-only edit leaves the frontmatter unchanged (still valid, since
the note started valid), so this is a no-op for the common case and
fail-closed for every case that touches the envelope, however old_str is
phrased.

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

ingestion/internal/mcp/tools_notes_test.go

@@ -141,6 +141,33 @@ func TestBrainEdit_RejectsAValueOnlyEditThatBreaksTheEnvelope(t *testing.T) {
 	assert.Contains(t, string(got), "claim_provenance: asserted", "rejected edit must leave the note byte-identical")
 }
 
+func TestBrainEdit_RejectsAnOldStrThatCrossesTheClosingFence(t *testing.T) {
+	// Found in round-2 review: the position-based gate required old_str to
+	// be fully CONTAINED in the extracted frontmatter block (which excludes
+	// both "---" fence lines). An old_str disambiguated by extending past
+	// the closing fence into the body — a natural choice, since notes.Edit
+	// requires old_str to be unique in the file — fails that containment
+	// test, so checkClaimEnvelope skips validation entirely while
+	// notes.Edit still applies the replace verbatim. Same invalid-enum
+	// outcome as the original two bypasses, reached a third way.
+	dir := newBrainRepo(t)
+	note := filepath.Join(dir, "wiki/homelab/facts/n.md")
+	require.NoError(t, os.WriteFile(note,
+		[]byte("---\ntitle: 'n'\nhalf_life: durable\napplies: universal\nclaim_provenance: asserted\n---\n\nbody\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\n---\n\nbody",
+		"new_str": "guessed\n---\n\nbody",
+	})
+	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")