feat(server): 会话上下文保留（LLM 历史 + 确定性兜底双保险）

解决用户删除草稿后说'再提交'丢失上下文的问题：

- steward.py 新增 _inject_recent_conversation_history：build_plan 前读最近 10 条对话注入 context_json
- steward_intent_agent.py 的 _build_messages 把 recent_history 暴露给模型，system prompt 加确认类话术引导
- 新建 steward_context_resume.py：should_resume_recent_task 检测'再提交'类话术 + state 有可恢复 flow，attach_resumed_task 从 state 恢复 task
- 两个 plan 入口（/plans 和 /plans/stream）都已接入双保险
- 后端 67 passed，端到端验证'上海出差→再提交'成功恢复 task

server/tests/test_steward_intent_agent.py

@@ -98,3 +98,58 @@ def test_steward_intent_system_prompt_mentions_query_intent_guidance() -> None:
     assert "query_travel_standard" in system_prompt
     assert "差旅" in system_prompt
     assert "住宿标准" in system_prompt
+
+
+def test_steward_intent_system_prompt_includes_conversation_history_guidance() -> None:
+    """system prompt 应包含'结合对话历史理解确认类话术'的引导。"""
+    from app.services import steward_intent_bootstrap  # noqa: F401
+
+    messages = StewardIntentAgent._build_messages(
+        StewardPlanRequest(message="再提交"),
+        base_date=__import__("datetime").date(2026, 6, 24),
+        canonical_fields=["location", "time_range"],
+    )
+    system_prompt = messages[0]["content"]
+    assert "recent_history" in system_prompt
+    assert "再提交" in system_prompt
+    assert "确认类话术" in system_prompt
+
+
+def test_steward_intent_context_payload_includes_recent_history() -> None:
+    """context_payload 应携带 recent_history 结构化字段（role + content）。"""
+    import json
+
+    request = StewardPlanRequest(
+        message="再提交",
+        context_json={
+            "recent_history": [
+                {"role": "user", "content": "2026-02-20 至 2026-02-23，去上海出差，火车"},
+                {"role": "assistant", "content": "好的，为您整理出差申请预览。"},
+                {"role": "user", "content": "直接提交"},
+                {"role": "assistant", "content": "检测到重复申请，已暂停提交。"},
+            ],
+        },
+    )
+    messages = StewardIntentAgent._build_messages(
+        request,
+        base_date=__import__("datetime").date(2026, 6, 24),
+        canonical_fields=["location", "time_range"],
+    )
+    user_payload = json.loads(messages[1]["content"])
+    assert "recent_history" in user_payload
+    assert len(user_payload["recent_history"]) == 4
+    assert user_payload["recent_history"][0]["role"] == "user"
+    assert "上海" in user_payload["recent_history"][0]["content"]
+
+
+def test_steward_intent_context_payload_omits_empty_recent_history() -> None:
+    """无 recent_history 时不应注入空列表。"""
+    import json
+
+    messages = StewardIntentAgent._build_messages(
+        StewardPlanRequest(message="你好"),
+        base_date=__import__("datetime").date(2026, 6, 24),
+        canonical_fields=["location"],
+    )
+    user_payload = json.loads(messages[1]["content"])
+    assert user_payload.get("recent_history", []) == []