fix(device-pool): scope primary key by host and thread through capability_tags

- pooled_devices primary key changed from device_id alone to
  (host_id, device_id), so two hosts reporting the same local
  device_id no longer crash sync_host_devices() with an uncaught
  sqlite3.IntegrityError.
- Device gained a capability_tags field so DevicePool.sync_host_devices()
  can actually populate PooledDevice.capability_tags from a real host
  sync instead of always falling back to an empty list.

openspec: device-pool capability, archived change cloud-runtime

cloud/pool.py

@@ -106,7 +106,7 @@ class DevicePool:
         synced_at: datetime,
     ) -> PooledDevice:
         raw_status = device.status if device.status in _HOST_REPORTED_STATUSES else "idle"
-        tags = list(getattr(device, "capability_tags", []) or [])
+        tags = list(device.capability_tags or [])
         return PooledDevice(
             device_id=device.id,
             host_id=host_id,