feat(agent): add LangGraph scaffold for assignment 1

- src/agent/state.py: minimal five-field AgentState
- src/agent/graph.py: START -> normalize_input -> reply -> END,
  compiled with InMemorySaver; reply emits <state>1002</state> prefix
- examples/langgraph_smoke.py: offline smoke, turns 1/2/1 verified
- test/agent/test_scaffold.py: turn accumulation, thread isolation,
  fixed-template assertions (3 passed)
- requirements.txt: pin langgraph==1.2.9

Known limit: InMemorySaver state disappears on process exit.

test/agent/test_scaffold.py

@@ -0,0 +1,56 @@
+"""作业 1 验收测试：LangGraph 最小图。
+
+通过标准（对应迁移计划作业 1）：
+- 同一会话（thread）轮数累计：1 → 2；
+- 不同会话相互隔离：新 thread 的轮数从 1 开始；
+- 不依赖 FastGPT 与真实模型，离线可重复执行。
+
+注意：节点实现（src/agent/graph.py 中的 TODO）完成前，
+本文件的测试会失败，属于预期状态。
+"""
+
+import pytest
+
+from src.agent.graph import build_graph
+
+
+async def ask(graph, thread_id: str, text: str) -> dict:
+    # 只传本轮变化的字段；对已有 checkpoint 的 thread 传完整初始状态，
+    # 其中的 turn_count=0 会覆盖已累计的值（LangGraph 输入按通道覆盖）。
+    return await graph.ainvoke(
+        {"session_id": thread_id, "input_text": text},
+        config={"configurable": {"thread_id": thread_id}},
+    )
+
+
+@pytest.mark.asyncio
+async def test_same_thread_accumulates_turns() -> None:
+    graph = build_graph()
+
+    first = await ask(graph, "thread-a", "你好")
+    second = await ask(graph, "thread-a", "发生了交通事故")
+
+    assert first["turn_count"] == 1
+    assert second["turn_count"] == 2
+
+
+@pytest.mark.asyncio
+async def test_different_threads_are_isolated() -> None:
+    graph = build_graph()
+
+    await ask(graph, "thread-a", "你好")
+    await ask(graph, "thread-a", "发生了交通事故")
+    other = await ask(graph, "thread-b", "你好")
+
+    assert other["turn_count"] == 1
+
+
+@pytest.mark.asyncio
+async def test_reply_uses_fixed_template_with_stage_prefix() -> None:
+    graph = build_graph()
+
+    result = await ask(graph, "thread-a", "你好")
+
+    assert result["stage_code"] == "1002"
+    assert result["response_text"].startswith("<state>1002</state>")
+    assert "你好" in result["response_text"]