feat(steward): 拦截业务无关输入返回 off_topic 计划

- schemas/steward.py：StewardPlanResponse 新增 suggested_prompts 字段
- steward_planner.py：新增 STEWARD_BUSINESS_SIGNAL_KEYWORDS 与
  _is_business_irrelevant_input 守卫，在 build_plan 入口前置；
  新增 _build_off_topic_plan 构造 plan_status=off_topic 的引导计划
- steward_intent_agent.py：system prompt 追加业务无关约束
- test_steward_planner.py：覆盖 123/你好/纯标点走 off_topic，
  并验证正常业务输入不受守卫影响

server/src/app/schemas/steward.py

@@ -128,6 +128,10 @@ class StewardPlanResponse(BaseModel):
     )
     candidate_flows: list[StewardCandidateFlow] = Field(default_factory=list, description="等待用户确认的候选流程快捷列表。")
     model_call_traces: list[dict[str, Any]] = Field(default_factory=list, description="模型工具调用轨迹。")
+    suggested_prompts: list[str] = Field(
+        default_factory=list,
+        description="当 plan_status 为 off_topic 等场景时，给用户的推荐话术示例。",
+    )
 
 
 class StewardSlotOption(BaseModel):