feat(mcp): expose brain_graph tool — neighbors, subgraph, path

Commit 3 of Track A. The MCP server now publishes a new tool that
opens the brain knowledge graph (entities + wikilink edges) for
external consumers (claude.ai connectors, gitea-mcp, agentsquad).

- tools_graph.go: brain_graph handler dispatches by op:
    neighbors  — 1-hop outgoing from slug, optional edge_type filter
    subgraph   — every reachable slug within depth hops (≤6)
    path       — shortest directed path src→dst within depth (≤8)
  Returns slug + entity metadata + edge_type + hop distance.

- server.go: handleCall routes "brain_graph" to brainGraph.

- handlers.go: tool descriptor with the op enum + per-op required
  fields documented in the description.

- server_test.go: TestServerToolsList expects brain_graph in the
  listing.

The tool returns an error when BRAIN_GRAPH_ENABLED is unset — same
shape as brain_answer when the answer LLM is unconfigured.

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

ingestion/internal/mcp/server_test.go

@@ -57,7 +57,7 @@ func TestServerToolsList(t *testing.T) {
 	assert.ElementsMatch(t, []string{
 		"brain_query", "brain_write", "brain_index", "brain_tunnel",
 		"brain_ingest_raw", "brain_ingest",
-		"brain_answer", "brain_classify", "session_log",
+		"brain_answer", "brain_classify", "brain_graph", "session_log",
 	}, names)
 }