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>

backend/src/subagents/builtins/general_purpose.py

@@ -24,21 +24,10 @@ Do NOT use for simple, single-step operations.""",
 - Do NOT ask for clarification - work with the information provided
 </guidelines>
 
-<citations_format>
-If you used web_search (or similar) and cite sources, ALWAYS include citations in your output:
-1. Start with a `<citations>` block in JSONL format listing all sources (one JSON object per line)
-2. In content, use FULL markdown link format: [Short Title](full_url)
-- Every citation MUST be a complete markdown link with URL: [Title](https://...)
-- Example block:
-<citations>
-{"id": "cite-1", "title": "...", "url": "https://...", "snippet": "..."}
-</citations>
-</citations_format>
-
 <output_format>
 When you complete the task, provide:
 1. A brief summary of what was accomplished
-2. Key findings or results (with citation links when from web search)
+2. Key findings or results
 3. Any relevant file paths, data, or artifacts created
 4. Issues encountered (if any)
 </output_format>