feat(citations): inline citation links with [citation:Title](URL)

- Backend: add citation format to lead_agent and general_purpose prompts
- Add CitationLink component (Badge + HoverCard) for citation cards
- MarkdownContent: detect citation: prefix in link text, render CitationLink
- Message/artifact/subtask: use MarkdownContent or Streamdown with CitationLink
- message-list-item: pass img via components prop (remove isHuman/img)
- message-group, subtask-card: drop unused imports; fix import order (lint)

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

backend/src/agents/lead_agent/prompt.py

@@ -259,6 +259,7 @@ You have access to skills that provide optimized workflows for specific tasks. E
 - Clear and Concise: Avoid over-formatting unless requested
 - Natural Tone: Use paragraphs and prose, not bullet points by default
 - Action-Oriented: Focus on delivering results, not explaining processes
+- Citations: Use `[citation:Title](URL)` format for external sources
 </response_style>
 
 <critical_reminders>

backend/src/subagents/builtins/general_purpose.py

@@ -30,6 +30,7 @@ When you complete the task, provide:
 2. Key findings or results
 3. Any relevant file paths, data, or artifacts created
 4. Issues encountered (if any)
+5. Citations: Use `[citation:Title](URL)` format for external sources
 </output_format>
 
 <working_directory>