fix: correct task scheduling and simplify agent internals

Fix queue cancellation, concurrency limits and worker status updates.

Use direct Explorer imports, split the Diagnoser loop into helpers and remove unused tool registration and prompt-builder code.

Correct model caching, endpoint selection and retry handling.

artemis/runtime/device_pool.py

@@ -436,7 +436,15 @@ class DevicePool:
             selects the first ready device so the task will queue on it.
             Returns None if no devices are attached.
         """
-        all_devs = self.list_devices()
+        return self._pick_device(self.list_devices(), preferred_serial)
+
+    async def select_device_async(self, preferred_serial: str | None = None) -> str | None:
+        """Select a device without blocking the caller on ADB enumeration."""
+        return self._pick_device(await self.list_devices_async(), preferred_serial)
+
+    def _pick_device(
+        self, all_devs: list[DeviceStatus], preferred_serial: str | None
+    ) -> str | None:
         ready_devs = [d for d in all_devs if d.state == "device"]
 
         if preferred_serial: