Post Message Stream

The desk's message turn (H-1, now THE message route — the non-streaming twin is gone; the web cut over). Appends the user's turn, runs the desk driver, and streams the reply PROSE token-by-token over SSE (delta events), then a terminal done event with the persisted messages. The pipeline phase feed (run SSE) is unchanged — it still drives 'what's happening'. [IP] the stream carries only assistant prose (run_agent_streaming's text deltas); the tool channel never leaves the server.

Recovery contract: the user message is persisted before the driver runs, and the driver turn runs on its OWN db session in a task that survives client disconnect — so a dropped stream is recovered by refetching the thread, never by re-sending (which would run the turn twice).