feat: add [TRIAGE_UPDATE] marker extraction and auto-PATCH (Phase 2)

- Add _parse_triage_update_marker() parser following existing marker pattern
- Add [TRIAGE_UPDATE] instructions to system prompt with grounding rules
- Add QuestionItem.options support in question parser
- Wire triage extraction into both main and branch-aware chat paths
- Auto-PATCH session: AI only fills null fields (manual edits win)
- Evidence items: AI appends only, never modifies existing
- Return triage_update in ChatMessageResponse for frontend header sync

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/api/endpoints/ai_sessions.py

@@ -308,7 +308,7 @@ async def send_chat_message(
             message = f"{message}\n\n[Attached document content]\n{doc_context}"
 
     try:
-        ai_content, suggested_flows, session, fork_metadata, actions_data, questions_data = await unified_chat_service.send_chat_message(
+        ai_content, suggested_flows, session, fork_metadata, actions_data, questions_data, triage_update_data = await unified_chat_service.send_chat_message(
             session_id=session_id,
             user_id=user_id,
             account_id=account_id,
@@ -353,6 +353,7 @@ async def send_chat_message(
         fork=fork_metadata,
         actions=actions_data,
         questions=questions_data,
+        triage_update=triage_update_data,
     )