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/__init__.py

@@ -2,7 +2,7 @@
 AI Agent 应用模块
 """
 
-from .agent.service import AIAgentService
+from .agent.agent_service import AIAgentService
 from app.main_graph.tools.graph_tools import AVAILABLE_TOOLS, TOOLS_BY_NAME
 
 __all__ = ["AIAgentService", "AVAILABLE_TOOLS", "TOOLS_BY_NAME"]