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/server.go

@@ -9,6 +9,8 @@ import (
 	"fmt"
 	"net/http"
 
+	"github.com/mathiasbq/hyperguild/ingestion/internal/graphstore"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/graphsync"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/pipeline"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/reranker"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/search"
@@ -42,6 +44,7 @@ type Server struct {
 	reranker  *reranker.Client      // nil = no rerank, BM25 top-10 → LLM
 	vector    search.VectorSearcher // nil = BM25-only retrieval
 	embedder  search.Embedder       // nil = BM25-only retrieval
+	graph     graphsync.Store       // nil = brain_graph and GraphRAG augmentation disabled
 }
 
 // NewServer constructs a Server bound to brainDir. pipelineCfg supplies the
@@ -73,6 +76,19 @@ func (s *Server) WithHybridRetrieval(v search.VectorSearcher, e search.Embedder)
 	return s
 }
 
+// WithGraph wires the brain entities + edges store so every successful
+// brain_write / brain_ingest / brain_tunnel re-indexes its written docs
+// into the graph, and so brain_graph + GraphRAG-augmented brain_answer
+// are available. nil disables graph features and is the legacy default.
+func (s *Server) WithGraph(g *graphstore.PGStore) *Server {
+	if g == nil {
+		s.graph = nil
+		return s
+	}
+	s.graph = g
+	return s
+}
+
 func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	// MCP streamable HTTP: GET establishes the SSE stream for server-to-client events.
 	if r.Method == http.MethodGet {