Server-sent events for the threads of every agent the caller may use

event: ready first, then event: thread (an AgentThreadEvent) whenever a thread is created, renamed, starts or finishes a reply, or is deleted. One stream follows every conversation, so a client need not open one per thread (shared threads and the caller's own private ones). agentId limits it to one agent; once=true sends ready and ends. A keep-alive comment comes every 15 seconds; reconnect when the stream ends (it does when the platform restarts).