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/nodes/react_nodes.py

@@ -325,7 +325,7 @@ def route_by_reasoning(state: MainGraphState) -> str:
     route = get_route_by_reasoning(reasoning_result)
 
     # 映射到我们的节点名称
-    # 注意：这些名称必须与 subgraph_builder.py 中定义的节点名称一致
+    # 注意：这些名称必须与 main_graph_builder.py 中定义的节点名称一致
     route_mapping = {
         "direct_response": "final_response",
         "retrieve_rag": "rag_retrieve",