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/handlers.go

@@ -109,6 +109,19 @@ func (s *Server) tools() []map[string]any {
 				"text": str("raw document text to classify (first 3000 chars used)"),
 			}),
 		},
+		{
+			"name":        "brain_graph",
+			"description": "Query the brain knowledge graph (entities + wikilink edges). Op selects the traversal: neighbors (1-hop outgoing from slug), subgraph (every reachable slug within depth hops), or path (shortest directed path src→dst). Returns slug + entity metadata + edge_type + hop distance.",
+			"inputSchema": schema([]string{"op"}, map[string]any{
+				"op":        enum("traversal kind", "neighbors", "subgraph", "path"),
+				"slug":      str("origin slug for op=neighbors or op=subgraph"),
+				"src":       str("source slug for op=path"),
+				"dst":       str("destination slug for op=path"),
+				"edge_type": str("optional edge type filter for op=neighbors (e.g. wikilink); empty matches all"),
+				"limit":     int_("max neighbors to return for op=neighbors, default 25"),
+				"depth":     int_("max traversal depth for op=subgraph (default 2, clamped to 6) and op=path (default 4, clamped to 8)"),
+			}),
+		},
 		{
 			"name":        "session_log",
 			"description": "Append a structured entry to brain/sessions/<session_id>.jsonl.",