Report agent status (heartbeat)

Example response

{
  "id": "clxyz456",
  "name": "My Agent",
  "description": "A helpful assistant",
  "avatarUrl": "https://example.com/avatar.png",
  "webhookUrl": "https://example.com/webhook",
  "lastActiveAt": "2025-01-01T12:00:00.000Z",
  "createdAt": "2025-01-01T00:00:00.000Z"
}