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/src/components/workspace/messages/message-list.tsx

@@ -26,7 +26,7 @@ import { StreamingIndicator } from "../streaming-indicator";
 
 import { MessageGroup } from "./message-group";
 import { MessageListItem } from "./message-list-item";
-import { SafeCitationContent } from "./safe-citation-content";
+import { MarkdownContent } from "./markdown-content";
 import { MessageListSkeleton } from "./skeleton";
 import { SubtaskCard } from "./subtask-card";
 
@@ -69,7 +69,7 @@ export function MessageList({
             const message = group.messages[0];
             if (message && hasContent(message)) {
               return (
-                <SafeCitationContent
+                <MarkdownContent
                   key={group.id}
                   content={extractContentFromMessage(message)}
                   isLoading={thread.isLoading}
@@ -89,7 +89,7 @@ export function MessageList({
             return (
               <div className="w-full" key={group.id}>
                 {group.messages[0] && hasContent(group.messages[0]) && (
-                  <SafeCitationContent
+                  <MarkdownContent
                     content={extractContentFromMessage(group.messages[0])}
                     isLoading={thread.isLoading}
                     rehypePlugins={rehypePlugins}