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/task_worker.py

@@ -45,6 +45,22 @@ def _html_to_text(html: str) -> str:
     return text.strip()
 
 
+def _live_model(default: str) -> str:
+    """Follow the conversation's live model selection when it is fresher.
+
+    The main agent writes the UI-selected model to the session's task dir
+    (shared with this MCP subprocess) on every set_model. Env is the
+    fallback for tasks dispatched before any switch happened.
+    """
+    env = os.environ.get("TASKS_DIR", "/tmp/tasks")
+    try:
+        with open(os.path.join(env, "model.txt")) as f:
+            chosen = f.read().strip()
+    except OSError:
+        return default
+    return chosen or default
+
+
 class TaskWorker:
     """Runs a background research/lookup task using the LLM + tools."""
 
@@ -65,7 +81,7 @@ class TaskWorker:
             or os.environ.get("GEMMA_BASE_URL")
             or "http://192.168.86.2:3005/api"
         )
-        model = (
+        model = _live_model(
             os.environ.get("LLM_MODEL")
             or os.environ.get("GEMMA_MODEL")
             or "qwen2.5-vl-3b"