Vibe Chat Stream

Streaming counterpart to /chat, for sessions on the agent pipeline.

A separate endpoint rather than content-negotiation on /chat: the legacy pipeline has nothing to stream (one model call, then a response), and teaching a 700-line handler to sometimes be a generator is how the fallback path stops being trustworthy.

A session not routed to the agent gets a single fallback frame and the client re-issues against /chat. That keeps the decision about WHICH pipeline runs in one place — should_use_agent — instead of duplicating it in the frontend.