refactor: 重命名文件更清晰，调整最大推理步数为10

- subgraph_builder.py → main_graph_builder.py
- service.py → agent_service.py
- 调整 max_steps 从 40 → 10
- 更新所有相关导入

backend/app/backend.py

@@ -15,7 +15,7 @@ from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import StreamingResponse
 from pydantic import BaseModel
 from langgraph.checkpoint.postgres.aio import AsyncPostgresSaver
-from .agent.service import AIAgentService
+from .agent.agent_service import AIAgentService
 from .agent.history import ThreadHistoryService
 from app.core.human_review import (
     ReviewManager,