feat(ingestion): implement brain_query MCP tool

Wraps the existing search.Query function. Same BM25 over
brain/knowledge/ and brain/wiki/ that the HTTP /query endpoint serves.
Plan note: handleCall switch replaces the single-line stub from Task 1
— no unknownToolError type to remove since Task 1 inlined the error.

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

ingestion/internal/mcp/server.go

@@ -113,7 +113,12 @@ func writeError(w http.ResponseWriter, id any, code int, msg string) {
 	})
 }
 
-// handleCall dispatches a tools/call. Stub for Task 1; expanded in later tasks.
+// handleCall dispatches a tools/call to the appropriate tool handler.
 func (s *Server) handleCall(ctx context.Context, name string, args json.RawMessage) (json.RawMessage, error) {
-	return nil, fmt.Errorf("unknown tool: %s", name)
+	switch name {
+	case "brain_query":
+		return s.brainQuery(ctx, args)
+	default:
+		return nil, fmt.Errorf("unknown tool: %s", name)
+	}
 }