feat(mcp): record and alert on a governance-file rewrite (#97) (#98)

brain_edit_root was gated but unobserved: on success it emitted a slog.Warn
and nothing else. That proves a rewrite was allowed, not that one happened —
and brain-sync commits it within 15 minutes authored `brain-sync`, so git
history cannot tell a CLAUDE.md rewrite apart from routine note traffic.

A successful brain_edit_root now emits an ntfy notification and an AuditEntry
through the same sink a capture already uses. notes.Result gains
PrevContentHash so both ends of the change are identifiable.

Path and hashes only in the notification body. Reserved at Internal, not
Confidential, because Confidential makes DegradingSink refuse to record while
loki is down rather than buffer. Observation is best-effort and never fails
the call; a failure to observe logs at ERROR.

Ordinary brain_edit on knowledge//wiki/ is deliberately not audited — git
history already carries the same before/after bytes with a diff.

Closes #97.

Co-authored-by: Mathias <mthbqv@gmail.com>

ingestion/internal/mcp/tools_notes_test.go

@@ -2,13 +2,22 @@
 package mcp_test
 
 import (
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
 	"encoding/json"
 	"net/http"
 	"net/http/httptest"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 
+	"github.com/mathiasbq/hyperguild/ingestion/internal/audit"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brainstore"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/capture"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/capturehttp"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/classification"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/mcp"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -125,3 +134,164 @@ func mustJSON(t *testing.T, v any) []byte {
 	require.NoError(t, err)
 	return b
 }
+
+// ---- hyperguild#97: a governance rewrite must be observed, not just gated ----
+
+// govSink captures audit records. Governance edits are rare, so the whole
+// history is kept and asserted on rather than sampled.
+type govSink struct {
+	entries  []capture.AuditEntry
+	outcomes []capture.AuditOutcome
+	recErr   error
+}
+
+func (s *govSink) Reserve(context.Context, classification.Level) (capture.AuditOutcome, error) {
+	return capture.AuditCentral, nil
+}
+
+func (s *govSink) Record(_ context.Context, e capture.AuditEntry, o capture.AuditOutcome) error {
+	s.entries = append(s.entries, e)
+	s.outcomes = append(s.outcomes, o)
+	return s.recErr
+}
+
+type govNotifier struct {
+	msgs []string
+	err  error
+}
+
+func (n *govNotifier) Notify(_ context.Context, msg string) error {
+	n.msgs = append(n.msgs, msg)
+	return n.err
+}
+
+// govServer wires capture (which is what re-derives the static principal) plus
+// the #97 observer, and returns the brain dir so the test can read the file.
+func govServer(t *testing.T) (*mcp.Server, string, *govSink, *govNotifier) {
+	t.Helper()
+	dir := newBrainRepo(t)
+	cfg, err := classification.Load(dir)
+	require.NoError(t, err)
+	svc := capture.NewService(brainstore.New(dir), capFakeTracker{}, cfg, audit.NewSlogSink(nil))
+
+	srv := mcp.NewServer(dir, nil, nil, nil)
+	srv.WithCapture(svc, nil, capStaticTok, "local-cli", capturehttp.NewOriginResolver(nil))
+	sink, notifier := &govSink{}, &govNotifier{}
+	srv.WithGovernanceObserver(sink, notifier)
+	return srv, dir, sink, notifier
+}
+
+// authCall is call() with an Authorization header, so the static principal is
+// present and brain_edit_root's gate opens.
+func authCall(t *testing.T, srv *mcp.Server, authz, name string, args map[string]any) map[string]any {
+	t.Helper()
+	req := httptest.NewRequest(http.MethodPost, "/mcp", body(t, map[string]any{
+		"jsonrpc": "2.0", "id": 1, "method": "tools/call",
+		"params": map[string]any{"name": name, "arguments": args},
+	}))
+	if authz != "" {
+		req.Header.Set("Authorization", authz)
+	}
+	rr := httptest.NewRecorder()
+	srv.ServeHTTP(rr, req)
+	var resp map[string]any
+	require.NoError(t, json.Unmarshal(rr.Body.Bytes(), &resp))
+	return resp
+}
+
+func sha256hex(s string) string {
+	sum := sha256.Sum256([]byte(s))
+	return hex.EncodeToString(sum[:])
+}
+
+func TestBrainEditRoot_RecordsAnAuditEntryNamingPrincipalPathAndBothHashes(t *testing.T) {
+	// The gate proves a rewrite was ALLOWED. Only a record proves one HAPPENED
+	// — and brain-sync will commit it within 15 minutes authored `brain-sync`,
+	// so git history cannot tell it apart from routine note traffic.
+	srv, dir, sink, _ := govServer(t)
+
+	resp := authCall(t, srv, "Bearer "+capStaticTok, "brain_edit_root", map[string]any{
+		"path": "CLAUDE.md", "old_str": "# rules", "new_str": "# rewritten",
+	})
+	require.NotContains(t, resp, "error", "%v", resp["error"])
+
+	require.Len(t, sink.entries, 1, "exactly one audit record per governance rewrite")
+	e := sink.entries[0]
+	assert.Equal(t, "local-cli", e.Principal)
+	assert.Contains(t, e.Items, "CLAUDE.md", "the record must name what was rewritten")
+
+	joined := strings.Join(e.SecurityEvents, " ")
+	assert.Contains(t, joined, sha256hex("# rules\n"), "before-hash must be recorded")
+	assert.Contains(t, joined, sha256hex("# rewritten\n"), "after-hash must be recorded")
+	assert.NotZero(t, e.Timestamp)
+
+	got, err := os.ReadFile(filepath.Join(dir, "CLAUDE.md"))
+	require.NoError(t, err)
+	assert.Equal(t, "# rewritten\n", string(got))
+}
+
+func TestBrainEditRoot_NotifiesWithPathAndHashesButNeverContent(t *testing.T) {
+	srv, _, _, notifier := govServer(t)
+
+	resp := authCall(t, srv, "Bearer "+capStaticTok, "brain_edit_root", map[string]any{
+		"path": "CLAUDE.md", "old_str": "# rules", "new_str": "# rewritten",
+	})
+	require.NotContains(t, resp, "error", "%v", resp["error"])
+
+	require.Len(t, notifier.msgs, 1)
+	msg := notifier.msgs[0]
+	assert.Contains(t, msg, "CLAUDE.md")
+	assert.Contains(t, msg, "local-cli")
+	assert.Contains(t, msg, sha256hex("# rules\n"))
+	assert.Contains(t, msg, sha256hex("# rewritten\n"))
+
+	// The notification travels to a phone through a third-party topic. A hash
+	// identifies a rewrite; the bytes would leak whatever was written.
+	assert.NotContains(t, msg, "# rewritten", "no file content in the notification body")
+	assert.NotContains(t, msg, "# rules")
+}
+
+func TestBrainEditRoot_RefusedProducesNoRecordNoNotificationNoWrite(t *testing.T) {
+	srv, dir, sink, notifier := govServer(t)
+
+	// No Authorization header → no static principal → the gate stays shut.
+	resp := authCall(t, srv, "", "brain_edit_root", map[string]any{
+		"path": "CLAUDE.md", "old_str": "# rules", "new_str": "# rewritten",
+	})
+	require.Contains(t, resp, "error")
+
+	assert.Empty(t, sink.entries, "a refusal is not a rewrite — recording one would cry wolf")
+	assert.Empty(t, notifier.msgs)
+	got, err := os.ReadFile(filepath.Join(dir, "CLAUDE.md"))
+	require.NoError(t, err)
+	assert.Equal(t, "# rules\n", string(got))
+}
+
+func TestBrainEditRoot_FailedEditProducesNoRecordNoNotification(t *testing.T) {
+	// Authorised but no match: the door opened and nothing went through it.
+	srv, _, sink, notifier := govServer(t)
+
+	resp := authCall(t, srv, "Bearer "+capStaticTok, "brain_edit_root", map[string]any{
+		"path": "CLAUDE.md", "old_str": "absent-from-the-file", "new_str": "x",
+	})
+	require.Contains(t, resp, "error")
+
+	assert.Empty(t, sink.entries)
+	assert.Empty(t, notifier.msgs)
+}
+
+func TestBrainEdit_OnOrdinaryNotesIsNotAudited(t *testing.T) {
+	// The decided asymmetry (#97): note edits are covered by git history, which
+	// carries the same before/after bytes with a diff. Governance files get the
+	// extra trail because they change what later agents obey, not because they
+	// are harder to reconstruct.
+	srv, _, sink, notifier := govServer(t)
+
+	resp := authCall(t, srv, "Bearer "+capStaticTok, "brain_edit", map[string]any{
+		"path": "wiki/homelab/facts/n.md", "old_str": "beta", "new_str": "BETA",
+	})
+	require.NotContains(t, resp, "error", "%v", resp["error"])
+
+	assert.Empty(t, sink.entries, "note edits must not fill the governance audit trail")
+	assert.Empty(t, notifier.msgs)
+}