Prewarm browser voice for an agent

Signals intent to start a browser voice call (for example, the pointer entered the microphone button) so the WebSocket connect and first turn find warm state. Authenticated exactly like /agents/{agentId}/speak: send the client token as Authorization: Bearer ct_…; the token agent allowlist and origin allowlist apply. Returns 204 as soon as the caller is authenticated and warms in the background. The call never bills, reserves quota, or meters usage. Warming is rate-limited per client token and agent; a limited call still returns 204 and does nothing.