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/main_graph/state.py

@@ -79,7 +79,7 @@ class MainGraphState:
     
     # ========== React 推理专用字段 ==========
     reasoning_step: int = 0  # 当前推理步数
-    max_steps: int = 40  # 最大推理步数（≤40）
+    max_steps: int = 10  # 最大推理步数（≤10）
     last_action: str = ""  # 上一步动作
     reasoning_history: List[Dict[str, Any]] = field(default_factory=list)  # 推理历史