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/tests/test_steward_planner.py

@@ -547,3 +547,69 @@ def test_steward_plan_endpoint_persists_application_and_reimbursement_state() ->
     assert state["flows"]["travel_reimbursement"]["fields"]["time_range"] == "2026-06-03"
     assert state["flows"]["travel_reimbursement"]["fields"]["expense_type"] == "transport"
     assert all("invented_field" not in flow["fields"] for flow in state["flows"].values())
+
+
+def test_steward_planner_returns_off_topic_for_business_irrelevant_input() -> None:
+    payload = StewardPlanRequest(
+        message="123",
+        client_now_iso="2026-06-04T09:30:00+08:00",
+    )
+
+    result = StewardPlannerService().build_plan(payload)
+
+    assert result.plan_status == "off_topic"
+    assert result.next_action == "none"
+    assert result.tasks == []
+    assert result.attachment_groups == []
+    assert result.confirmation_groups == []
+    assert result.candidate_flows == []
+    assert result.planning_source == "rule_fallback"
+    assert len(result.suggested_prompts) == 3
+    assert result.thinking_events[0].stage == "off_topic"
+
+
+def test_steward_planner_returns_off_topic_for_pure_greeting() -> None:
+    payload = StewardPlanRequest(
+        message="你好",
+        client_now_iso="2026-06-04T09:30:00+08:00",
+    )
+
+    result = StewardPlannerService().build_plan(payload)
+
+    assert result.plan_status == "off_topic"
+    assert result.next_action == "none"
+    assert result.tasks == []
+    assert result.candidate_flows == []
+    assert result.planning_source == "rule_fallback"
+    assert len(result.suggested_prompts) == 3
+    assert result.thinking_events[0].stage == "off_topic"
+
+
+def test_steward_planner_returns_off_topic_for_pure_punctuation() -> None:
+    payload = StewardPlanRequest(
+        message="??? !!!",
+        client_now_iso="2026-06-04T09:30:00+08:00",
+    )
+
+    result = StewardPlannerService().build_plan(payload)
+
+    assert result.plan_status == "off_topic"
+    assert result.next_action == "none"
+    assert result.tasks == []
+    assert result.candidate_flows == []
+    assert result.planning_source == "rule_fallback"
+    assert len(result.suggested_prompts) == 3
+    assert result.thinking_events[0].stage == "off_topic"
+
+
+def test_steward_planner_preserves_normal_business_flow_after_guard() -> None:
+    payload = StewardPlanRequest(
+        message="我要报销昨天的交通费",
+        client_now_iso="2026-06-04T09:30:00+08:00",
+    )
+
+    result = StewardPlannerService().build_plan(payload)
+
+    assert result.plan_status != "off_topic"
+    assert len(result.tasks) >= 1
+    assert [task.task_type for task in result.tasks] == ["reimbursement"]