Begin an agent-driven signup: submit an email, and Bubbles emails a verification code. The response is identical whether or not an account already exists (it never reveals whether the email is registered before the code is verified — the unified flow logs in existing users and signs up new ones). Returns a signup_session to pass to the next steps and relay_to_user text to show the user. Call again to resend. A malformed/disallowed/temporary email is rejected with 400 invalid_request. Per-IP throttled.