feat(standalone): 新增单机模式，KILOSTAR_MODE=standalone 时去掉 Ray 依赖

通过 StandaloneProxy 适配层让 .remote() 调用在单机模式下透明降级为
asyncio 协程调用，7 个 Actor 和 workflow task 均可在纯 asyncio 环境运行，
启动快、资源占用低。分布式模式行为完全不变。

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

kilostar/core/work/workflow/workflow_engine.py

@@ -36,7 +36,7 @@ import datetime
 from dataclasses import dataclass
 from typing import Any, Awaitable, Callable, Dict, List, Optional
 
-import ray
+from kilostar.utils.standalone_proxy import remote_task, _STANDALONE
 from pydantic import BaseModel, Field
 from pydantic_graph import BaseNode, End, Graph, GraphRunContext
 from pydantic_graph.persistence import BaseStatePersistence
@@ -519,7 +519,7 @@ async def resume_workflow_graph(
     return final_output
 
 
-@ray.remote
+@remote_task
 def run_workflow_task(
     workflow_data: dict, trace_id: str, resume_only: bool = False
 ):
@@ -575,4 +575,7 @@ def run_workflow_task(
                     workflow_data, trace_id, persistence=persistence
                 )
 
-    asyncio.run(_entry())
+    if _STANDALONE:
+        return _entry()
+    else:
+        asyncio.run(_entry())