feat(graph): wire graphsync into MCP write/ingest/tunnel handlers

Commit 2 of Track A. Service stays a no-op until BRAIN_GRAPH_ENABLED=
true; flipping it on creates the schema (idempotent), starts indexing
every successful write, and optionally backfills the existing brain
dir.

- internal/graphsync: best-effort wrapper around graph.Extract +
  graphstore. IndexDoc reads docPath under brainDir, parses, upserts
  entity + replaces edges. BackfillFromBrainDir walks wiki/ +
  knowledge/. Both are no-ops on nil store so callers wire
  unconditionally.

- mcp.Server gains WithGraph builder + graphsync.Store field.
  brain_write, brain_ingest, brain_ingest_raw, brain_tunnel call
  indexInGraph after success — failures slog.Warn but never
  propagate (graph is augmentation, not correctness).

- cmd/server gates the wiring on BRAIN_GRAPH_ENABLED=true (default
  off so first rollout doesn't surprise). BRAIN_GRAPH_BACKFILL=true
  triggers a one-shot walk of the brain dir on boot.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ingestion/internal/graphsync/graphsync_test.go

@@ -0,0 +1,134 @@
+package graphsync
+
+import (
+	"context"
+	"errors"
+	"os"
+	"path/filepath"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/graph"
+)
+
+// fakeStore captures the calls IndexDoc / BackfillFromBrainDir made.
+type fakeStore struct {
+	mu       sync.Mutex
+	upserts  []graph.Entity
+	replaces map[string][]graph.Edge
+	deletes  []string
+	failOn   string // upsert fails when entity slug == failOn
+}
+
+func newFakeStore() *fakeStore {
+	return &fakeStore{replaces: make(map[string][]graph.Edge)}
+}
+
+func (f *fakeStore) UpsertEntity(_ context.Context, e graph.Entity) error {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	if f.failOn != "" && e.Slug == f.failOn {
+		return errors.New("synthetic failure")
+	}
+	f.upserts = append(f.upserts, e)
+	return nil
+}
+
+func (f *fakeStore) ReplaceEdgesForDoc(_ context.Context, docPath string, edges []graph.Edge) error {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.replaces[docPath] = append([]graph.Edge(nil), edges...)
+	return nil
+}
+
+func (f *fakeStore) DeleteByDoc(_ context.Context, docPath string) error {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.deletes = append(f.deletes, docPath)
+	return nil
+}
+
+func writeBrain(t *testing.T, brainDir, relPath, body string) {
+	t.Helper()
+	full := filepath.Join(brainDir, filepath.FromSlash(relPath))
+	require.NoError(t, os.MkdirAll(filepath.Dir(full), 0o755))
+	require.NoError(t, os.WriteFile(full, []byte(body), 0o644))
+}
+
+func TestIndexDoc_UpsertsEntityAndEdges(t *testing.T) {
+	tmp := t.TempDir()
+	writeBrain(t, tmp, "wiki/concepts/foo.md", `---
+title: Foo
+---
+# Foo
+Linking to [[bar]] and [[baz|Baz]].
+`)
+	fs := newFakeStore()
+	require.NoError(t, IndexDoc(context.Background(), fs, tmp, "wiki/concepts/foo.md"))
+
+	require.Len(t, fs.upserts, 1)
+	assert.Equal(t, "foo", fs.upserts[0].Slug)
+	assert.Equal(t, "concept", fs.upserts[0].Type)
+
+	edges := fs.replaces["wiki/concepts/foo.md"]
+	require.Len(t, edges, 2)
+	assert.Equal(t, "bar", edges[0].DstSlug)
+	assert.Equal(t, "baz", edges[1].DstSlug)
+}
+
+func TestIndexDoc_NoopOnNilStore(t *testing.T) {
+	require.NoError(t, IndexDoc(context.Background(), nil, "anywhere", "foo.md"))
+}
+
+func TestIndexDoc_NoopOnEmptyRelPath(t *testing.T) {
+	fs := newFakeStore()
+	require.NoError(t, IndexDoc(context.Background(), fs, "anywhere", ""))
+	assert.Empty(t, fs.upserts)
+}
+
+func TestIndexDoc_ErrorsOnMissingFile(t *testing.T) {
+	fs := newFakeStore()
+	err := IndexDoc(context.Background(), fs, t.TempDir(), "wiki/nope.md")
+	require.Error(t, err)
+}
+
+func TestIndexDoc_SurfacesStoreFailure(t *testing.T) {
+	tmp := t.TempDir()
+	writeBrain(t, tmp, "wiki/concepts/boom.md", "# Boom\n")
+	fs := newFakeStore()
+	fs.failOn = "boom"
+	err := IndexDoc(context.Background(), fs, tmp, "wiki/concepts/boom.md")
+	require.Error(t, err)
+}
+
+func TestBackfillFromBrainDir_WalksWikiAndKnowledge(t *testing.T) {
+	tmp := t.TempDir()
+	writeBrain(t, tmp, "wiki/concepts/foo.md", "# Foo\n[[bar]]\n")
+	writeBrain(t, tmp, "wiki/entities/bar.md", "# Bar\n")
+	writeBrain(t, tmp, "knowledge/legacy.md", "# Legacy [[foo]]\n")
+	// non-markdown file should be skipped
+	writeBrain(t, tmp, "wiki/concepts/skip.txt", "ignore me")
+
+	fs := newFakeStore()
+	n, err := BackfillFromBrainDir(context.Background(), fs, tmp)
+	require.NoError(t, err)
+	assert.Equal(t, 3, n)
+	assert.Len(t, fs.upserts, 3)
+}
+
+func TestBackfillFromBrainDir_TolerantOfMissingDirs(t *testing.T) {
+	tmp := t.TempDir()
+	fs := newFakeStore()
+	n, err := BackfillFromBrainDir(context.Background(), fs, tmp)
+	require.NoError(t, err)
+	assert.Equal(t, 0, n)
+}
+
+func TestBackfillFromBrainDir_NilStoreNoop(t *testing.T) {
+	n, err := BackfillFromBrainDir(context.Background(), nil, t.TempDir())
+	require.NoError(t, err)
+	assert.Equal(t, 0, n)
+}