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/mcp/handlers.go

@@ -12,6 +12,7 @@ import (
 	"github.com/mathiasbq/hyperguild/ingestion/internal/api"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/brain"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/extract"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/graphsync"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/pipeline"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/search"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/session"
@@ -194,9 +195,23 @@ func (s *Server) brainWrite(ctx context.Context, args json.RawMessage) (json.Raw
 			slog.Warn("brain_write: auto-tunnel failed", "src", relPath, "err", err)
 		}
 	}
+	s.indexInGraph(ctx, "brain_write", relPath)
 	return json.Marshal(map[string]string{"path": relPath})
 }
 
+// indexInGraph is a best-effort wrapper around graphsync.IndexDoc that
+// logs failures but never propagates them — the underlying write/ingest
+// has already succeeded and the graph is an augmentation, not a
+// correctness invariant.
+func (s *Server) indexInGraph(ctx context.Context, op, relPath string) {
+	if s.graph == nil || relPath == "" {
+		return
+	}
+	if err := graphsync.IndexDoc(ctx, s.graph, s.brainDir, relPath); err != nil {
+		slog.Warn(op+": graph index failed", "path", relPath, "err", err)
+	}
+}
+
 type brainTunnelArgs struct {
 	Source string `json:"source"`
 	Target string `json:"target"`
@@ -213,6 +228,8 @@ func (s *Server) brainTunnel(ctx context.Context, args json.RawMessage) (json.Ra
 	if err := brain.WriteTunnel(s.brainDir, a.Source, a.Target); err != nil {
 		return nil, fmt.Errorf("tunnel: %w", err)
 	}
+	s.indexInGraph(ctx, "brain_tunnel", a.Source)
+	s.indexInGraph(ctx, "brain_tunnel", a.Target)
 	return json.Marshal(map[string]string{"status": "ok"})
 }
 
@@ -268,6 +285,11 @@ func (s *Server) brainIngestRaw(ctx context.Context, args json.RawMessage) (json
 	if warnings == nil {
 		warnings = []string{}
 	}
+	if !a.DryRun {
+		for _, p := range pages {
+			s.indexInGraph(ctx, "brain_ingest_raw", p)
+		}
+	}
 	return json.Marshal(map[string]any{"pages": pages, "warnings": warnings})
 }
 
@@ -358,6 +380,11 @@ func (s *Server) runIngest(ctx context.Context, content, source string, dryRun b
 	if pages == nil {
 		pages = []string{}
 	}
+	if !dryRun {
+		for _, p := range pages {
+			s.indexInGraph(ctx, "brain_ingest", p)
+		}
+	}
 	warnings := result.Warnings
 	if warnings == nil {
 		warnings = []string{}