chore: 移除所有 Citations 相关逻辑，为后续重构做准备

- Backend: 删除 lead_agent / general_purpose 中的 citations_format 与引用相关 reminder；artifacts 下载不再对 markdown 做 citation 清洗，统一走 FileResponse，保留 Response 用于二进制 inline
- Frontend: 删除 core/citations 模块、inline-citation、safe-citation-content；新增 MarkdownContent 仅做 Markdown 渲染；消息/artifact 预览与复制均使用原始 content
- i18n: 移除 citations 命名空间（loadingCitations、loadingCitationsWithCount）
- 技能与 demo: 措辞改为 references，demo 数据去掉 <citations> 块
- 文档: 更新 CLAUDE/AGENTS/README 描述，新增按文件 diff 的代码变更总结

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/CLAUDE.md

@@ -30,7 +30,7 @@ Frontend (Next.js) ──▶ LangGraph SDK ──▶ LangGraph Backend (lead_age
                                               └── Tools & Skills
 ```
 
-The frontend is a stateful chat application. Users create **threads** (conversations), send messages, and receive streamed AI responses. The backend orchestrates agents that can produce **artifacts** (files/code), **todos**, and **citations**.
+The frontend is a stateful chat application. Users create **threads** (conversations), send messages, and receive streamed AI responses. The backend orchestrates agents that can produce **artifacts** (files/code) and **todos**.
 
 ### Source Layout (`src/`)