refactor(server): steward 决策链路改用 LangGraph 编排

- 新增 StewardGraphPlannerService，用 LangGraph 状态图编排意图识别→流程判断→模型/规则分支→兜底，替代原 planner 内线性调用
- 新增 StewardGraphRuntimeService 编排运行时决策与槽位决策；StewardActionContracts/Executor 统一动作合约与执行
- steward_intent_agent/application_fact_resolver/runtime_chat 适配图执行器，config 暴露图相关开关
- pyproject/uv.lock 新增 langgraph 依赖
- 新增 graph_planner/graph_runtime/action_executor 测试，更新 intent_agent/planner/fact_resolver/runtime_chat/reimbursement 测试

server/src/app/core/config.py

@@ -73,6 +73,7 @@ class Settings(BaseSettings):
     onlyoffice_backend_url: str = Field(default="", alias="ONLYOFFICE_BACKEND_URL")
     onlyoffice_jwt_secret: str = Field(default="", alias="ONLYOFFICE_JWT_SECRET")
     hermes_agent_shared_token: str = Field(default="", alias="HERMES_AGENT_SHARED_TOKEN")
+    steward_agent_runtime: str = Field(default="langgraph", alias="STEWARD_AGENT_RUNTIME")
 
     log_level: str = Field(default="INFO", alias="LOG_LEVEL")
     log_dir: str = Field(default="logs", alias="LOG_DIR")