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/utils/ray_hook.py

@@ -11,12 +11,11 @@
 #  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 #  See the License for the specific language governing permissions and
 #  limitations under the License.
-import os
 import time
 from functools import lru_cache
 from typing import Any, Dict
 
-_STANDALONE = os.environ.get("KILOSTAR_MODE", "distributed") == "standalone"
+from kilostar.utils.standalone_proxy import _STANDALONE
 
 if not _STANDALONE:
     import ray