Improve local agent chat and device health detection

device/manager.py

@@ -122,6 +122,20 @@ class DeviceManager:
             self._drivers.pop(device_id, None)
             self._set_status(device_id, "offline" if offline else "error")
 
+    def probe(self, device_id: str) -> bool:
+        """Check whether an established driver is still reachable."""
+        with self._lock:
+            self._device(device_id)
+            driver = self._drivers.get(device_id)
+            if driver is None:
+                return False
+        try:
+            driver.screenshot()
+        except Exception:
+            self.mark_error(device_id, offline=True)
+            return False
+        return True
+
     def active_driver(self, device_id: str | None = None) -> Driver:
         with self._lock:
             if device_id is None: