refactor(standalone): 收敛 _STANDALONE 判断到 standalone_proxy 单一来源

各模块不再各自读 os.environ，统一从 standalone_proxy._STANDALONE 导入，
环境变量只在一处决定模式，减少散落的重复逻辑。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

kilostar/worker_cluster/worker_cluster.py

@@ -12,14 +12,12 @@
 #  See the License for the specific language governing permissions and
 #  limitations under the License.
 
-import os
 import time
 import asyncio
 from collections import OrderedDict
-from kilostar.utils.standalone_proxy import actor_class
+from kilostar.utils.standalone_proxy import actor_class, _STANDALONE
 from kilostar.utils.ray_hook import ray_actor_hook
 
-_STANDALONE = os.environ.get("KILOSTAR_MODE", "distributed") == "standalone"
 if _STANDALONE:
     from asyncio import Queue
 else: