postAnchor method
Writes the live box anchor of runId (kind = run, 09 5.4): an agent message without
text behind the triggering message, then its run_id as message.updated.
Implementation
String postAnchor(String agentId, String runId) {
final run = world.runs[runId]!;
final created = world.createMessage(agentId, run['thread_id']! as String, {
'client_msg_id': world.nextId(),
'content_md': '',
'kind': 'run',
}, origin: 'agent');
final id = created.message['id']! as String;
world.updateMessageFields(id, {'run_id': runId}, actor: agentId);
return id;
}