Agent WebSocket

A bidirectional session with an Atoms agent. Open the connection with your API key and an `agent_id`; the server creates a session, bridges audio and transcripts to the STT/LLM/TTS pipeline, and streams events back until the client closes or the server ends the session. Post-call, the full conversation (transcript + recording) is available at `GET /atoms/v1/conversation/{callId}` using the `call_id` from `session.created`.