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.

agent/agent.py

@@ -750,6 +750,20 @@ def build_mcp_toolsets(user_profile: dict, task_dir: str) -> list[mcp.MCPToolset
     return toolsets
 
 
+def _sync_task_model(task_dir: str, model: str) -> None:
+    """Record the live model choice where background task workers can read it.
+
+    The dispatch MCP subprocess inherits its environment at spawn time, so it
+    can never see later UI model switches. The worker instead reads
+    <task_dir>/model.txt (falling back to LLM_MODEL env) when it starts a task.
+    """
+    try:
+        os.makedirs(task_dir, exist_ok=True)
+        with open(os.path.join(task_dir, "model.txt"), "w") as f:
+            f.write((model or "").strip())
+    except OSError as e:
+        logger.warning("failed to record selected model for background tasks: %s", e)
+
 class VoiceAssistant(Agent):
     """The conversational agent. LLM is the brain; STT/TTS are senses."""
 
@@ -824,6 +838,10 @@ async def handle_job(ctx: JobContext) -> None:
     # into it for live model switching.
     voice_assistant = VoiceAssistant(user_profile, task_dir)
 
+    # Background task workers (dispatch MCP subprocess) read this file so
+    # they use the same model as the conversation, even after live switches.
+    _sync_task_model(task_dir, voice_assistant.llm.model)
+
     # Publish the chat-capable model list so the web UI can offer a picker.
     async def _publish_models() -> None:
         models = await list_available_models()
@@ -1009,6 +1027,7 @@ async def handle_job(ctx: JobContext) -> None:
                             ack, reliable=True, topic="voice-control"
                         )
                     )
+                    _sync_task_model(task_dir, llm.model)
             except Exception as e:  # noqa: BLE001
                 logger.warning("Failed to update model: %s", e)