Onboarding Help Stream

Streaming sibling of POST /onboarding/help (SSE).

Two turn shapes behind one endpoint, chosen by ONBOARDING_ASSISTANT_ENABLED:

• assistant (default) — the tool-enabled turn (:func:onboarding_assistant_stream). Frames: tool_call / tool_result as the loop runs, draft_patch {"patch"} when the assistant proposes a change for the user to apply, then the same terminal done (plus patches). The answer does NOT arrive in delta frames — the gateway has no tool-calling stream.
• help (fallback) — the original text-streaming path, unchanged: event: delta {"text"} per visible chunk (the suggestions marker is held back server-side).

Either way one terminal event: done {"answer", "suggestions"} is authoritative — the same split as the plain endpoint — or event: error {"msg"}. : ping comments go out while the model is silent (adaptive thinking precedes the first token). Same gates as the plain endpoint — eligibility + per-user ONBOARDING_HELP_LIMIT, all enforced BEFORE the stream opens; 404 when the help chat or streaming is disabled (the app then falls back to the request/response endpoint). No DB, no conversation claim — unlike the CFO stream there is nothing to persist or lock, so a client disconnect simply closes the upstream stream.