fix: background tasks follow the live model selection

TaskWorker resolved its model from LLM_MODEL env (frozen at MCP spawn),
so after a live set_model switch in the UI, dispatched background tasks
still asked for the old model. On local-vl-models (llama-swap
--models-max 1) that forces a swap against the loaded conversation
model; a force-kill mid-load surfaces as 400 'model failed to load'.

The agent now writes the selected model to <task_dir>/model.txt at
session start and on every switch; TaskWorker reads it with the env
value as fallback.

AGENTS.md

@@ -216,6 +216,7 @@ curl -s "https://eastus.tts.speech.microsoft.com/cognitiveservices/v1" \
 - **Azure TTS uses SSML, not JSON.** The REST endpoint requires `Content-Type: application/ssml+xml`. The LiveKit Azure plugin handles this internally.
 - **Voice changes are live.** The web UI sends a data message to the agent; the agent calls `tts.update_options(voice=...)` without restarting. The agent filters data messages by topic ("voice-control") before processing.
 - **Model changes are live too.** The agent fetches the chat-capable model list (`HopeLLM` filters out embedding/reranker ids) and publishes `{type: "models", models, selected}`; the UI's settings sheet renders a picker and sends `{type: "set_model", model}`, which calls `HopeLLM.update_options(model=...)` (mutates `self._opts.model`) and acks with `{type: "model_changed", model}`. The choice persists in the browser's localStorage.
+- **Background tasks follow the live model.** The dispatch MCP subprocess inherits its env at spawn, so it cannot see later `set_model` switches. The agent writes the selected model to `<task_dir>/model.txt` at session start and on every switch; `TaskWorker` reads that file (falling back to `LLM_MODEL` env) when starting a task. Without this, a switched session (e.g. gemma-4-e4b) dispatches tasks against the stale env model (qwen2.5-vl-3b), which llama-swap on `local-vl-models` (`--models-max 1`) cannot load without killing the conversation model mid-swap.
 - **All supervisord programs log to /dev/stdout** so `docker compose logs -f voice` shows everything. Individual process logs are no longer written to files.
 - **Hope Music stays external.** `hope-media-music` owns yt-dlp, ffmpeg, the
   library volume, streaming endpoints, and the player UI. Do not add those