feat(agent): domain events and transition whitelist validation (assignment 3)

Architecture (mirrors workflow/20261016 chatNode structure):
- events.py: client markers parsed to typed ClientEvent (sources
  verified against workflow JSON); plain text like '拍好了' is not an event
- transitions.py: LEGAL_TRANSITIONS whitelist drafted from chatNode
  prompts and Condition edges (16 entries, each with source_ref);
  is_legal_transition rejects unlisted/terminal-violating moves
- graph.py: multi-node graph (collect/single_photo/dual_photo/...
  matching workflow chatNodes), route_by_stage conditional edges,
  _fake_llm_propose simulates LLM stage-code proposals (assignment 4
  replaces with real LLM); code only keeps context bookkeeping
  (no_reply_count) and validation
- state.py: expanded domain fields (pending_question, accident/
  person fields, clarify/no-reply counts, turn_patch, state_version)

Key finding: 单车拍照#6/双车拍照#7 are browser_addon branches ending
at 0000, so 2005/2015 allow both 300x (direct) and 0000.

Verified: pytest 87 passed (24 transition/event tests newly green).
Known limit: stage proposals come from a fake LLM; per-stage scripts
and real LLM integration are assignments 4-5.

test/agent/test_events.py

@@ -0,0 +1,41 @@
+"""作业 3 事件解析测试。
+
+来源：workflow/20261016/事故信息采集2025118.json 中的客户端标记。
+关键规则：普通自然语言（如“拍好了”“继续”）不是事件。
+"""
+
+import pytest
+
+from src.agent.events import ClientEvent, parse_client_event
+
+
+@pytest.mark.parametrize(
+    ("text", "expected"),
+    [
+        ("【开始】", ClientEvent.SESSION_START),
+        ("【继续办理】", ClientEvent.CONTINUE),
+        ("【拍摄完成】", ClientEvent.PHOTO_DONE),
+        ("【用户无回复】", ClientEvent.NO_REPLY),
+        ("【客户端连续3次拍摄识别失败：图片过于模糊】", ClientEvent.PHOTO_FAILED_3X),
+        ("【客户端连续3次拍摄识别失败：摄像头启动失败】", ClientEvent.PHOTO_FAILED_3X),
+        ("【客户端连续3次拍摄识别失败：未识别到完整车牌】", ClientEvent.PHOTO_FAILED_3X),
+        ("  【拍摄完成】  ", ClientEvent.PHOTO_DONE),  # 容忍首尾空白
+    ],
+)
+def test_markers_parse_to_events(text, expected):
+    assert parse_client_event(text) is expected
+
+
+@pytest.mark.parametrize(
+    "text",
+    [
+        "拍好了",            # 普通“拍好了”不等于拍摄完成事件
+        "我已经拍完了",
+        "继续",
+        "你好",
+        "",
+        "   ",
+    ],
+)
+def test_plain_text_is_not_an_event(text):
+    assert parse_client_event(text) is None