curl -X POST "https://app.talkover.ai/api/v1/agents/agent-uuid-1/callback" \
  -H "Authorization: Bearer talq_your_environment_token_here" \
  -H "Content-Type: application/json" \
  -d '{
    "source_agent_ids": ["agent-uuid-2", "agent-uuid-3"]
  }'

{
  "success": true,
  "data": {
    "id": "agent-uuid-1",
    "name": "Callback Handler",
    "direction": "inbound",
    "callback_config": {
      "source_agent_ids": ["agent-uuid-2", "agent-uuid-3"]
    }
  }
}

{
  "success": false,
  "message": "Agent must have direction=inbound to serve as a callback",
  "code": "INVALID_AGENT_DIRECTION"
}