feat(ingestion): implement session_log MCP tool

Appends a JSON line to brainDir/sessions/<session_id>.jsonl using the
session package copied in Task 2. Required for upcoming pass-rate
logging.

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

ingestion/internal/mcp/server.go

@@ -124,6 +124,8 @@ func (s *Server) handleCall(ctx context.Context, name string, args json.RawMessa
 		return s.brainIngestRaw(ctx, args)
 	case "brain_ingest":
 		return s.brainIngest(ctx, args)
+	case "session_log":
+		return s.sessionLog(ctx, args)
 	default:
 		return nil, fmt.Errorf("unknown tool: %s", name)
 	}