Merge upstream/experimental and resolve conflicts; citations + path_utils + mode-hover

## 冲突解决 (Resolve conflicts)
- input-box.tsx: 保留 ModeHoverGuide 包裹的模式选择器（PR #26 的 mode-hover-guide）
- message-group.tsx: 保留 getCleanContent / hasCitationsBlock / useParsedCitations
- message-list-item.tsx: 保留 useParsedCitations，移除重复的 MessageLink（使用 CitationAwareLink）
- artifact-file-detail.tsx: 保留 CitationAwareLink、useParsedCitations、contentWithoutCitationsFromParsed
- artifacts.py: 保留 path_utils 与 _extract_citation_urls + remove_citations_block 精简实现
- citations/index.ts: 保留并补充 contentWithoutCitationsFromParsed 导出
- en-US.ts: 保留 Ultra 模式描述 "Reasoning, planning and execution with subagents..."
- zh-CN.ts: 保留「超级」标签，描述保留「思考、计划并执行，可调用子代理分工协作...」

## PR #26 代码改动汇总

### 1. Citations（引用）
- lead_agent prompt: 增加 Web search 与子代理合成时的 citation 提示
- general_purpose: 子代理 system prompt 增加 <citations_format> 说明
- frontend utils: 新增 contentWithoutCitationsFromParsed，removeAllCitations 基于单次解析
- frontend artifact: 使用 contentWithoutCitationsFromParsed(parsed) 避免对同一内容解析两次
- backend artifacts: _extract_citation_urls + remove_citations_block，json 提到顶部

### 2. path_utils（路径解析）
- 新增 backend/src/gateway/path_utils.py：resolve_thread_virtual_path，防 path traversal
- artifacts.py / skills.py：删除内联路径解析，统一使用 path_utils

### 3. Mode hover guide
- input-box: 模式选择器外包 ModeHoverGuide，悬停展示模式说明

### 4. i18n
- en: ultraModeDescription 与 zh: ultraMode / ultraModeDescription 与上游对齐并保留 PR 文案

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

frontend/src/components/workspace/artifacts/artifact-file-detail.tsx

@@ -34,7 +34,11 @@ import { CodeEditor } from "@/components/workspace/code-editor";
 import { useArtifactContent } from "@/core/artifacts/hooks";
 import { urlOfArtifact } from "@/core/artifacts/utils";
 import type { Citation } from "@/core/citations";
-import { removeAllCitations, useParsedCitations } from "@/core/citations";
+import {
+  contentWithoutCitationsFromParsed,
+  removeAllCitations,
+  useParsedCitations,
+} from "@/core/citations";
 import { useI18n } from "@/core/i18n/hooks";
 import { installSkill } from "@/core/skills/api";
 import { streamdownPlugins } from "@/core/streamdown";
@@ -96,12 +100,10 @@ export function ArtifactFileDetail({
   );
   const cleanContent =
     language === "markdown" && content ? parsed.cleanContent : (content ?? "");
-  const contentWithoutCitations = useMemo(() => {
-    if (language === "markdown" && content) {
-      return removeAllCitations(content);
-    }
-    return content;
-  }, [content, language]);
+  const contentWithoutCitations =
+    language === "markdown" && content
+      ? contentWithoutCitationsFromParsed(parsed)
+      : (content ?? "");
 
   const [viewMode, setViewMode] = useState<"code" | "preview">("code");
   const [isInstalling, setIsInstalling] = useState(false);