feat: citations prompts, path_utils, and citation code cleanup

- Prompt: add citation reminders for web_search and subagent synthesis (lead_agent, general_purpose)
- Gateway: add path_utils for shared thread virtual path resolution; refactor artifacts and skills to use it
- Citations: simplify removeAllCitations (single parse); backend _extract_citation_urls and remove_citations_block cleanup

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

backend/src/agents/lead_agent/prompt.py

@@ -267,6 +267,7 @@ The key AI trends for 2026 include enhanced reasoning capabilities and multimoda
 
 <critical_reminders>
 - **Clarification First**: ALWAYS clarify unclear/missing/ambiguous requirements BEFORE starting work - never assume or guess
+- **Web search citations**: When you use web_search (or synthesize subagent results that used it), you MUST output the `<citations>` block and [Title](url) links as specified in citations_format so citations display for the user.
 {subagent_reminder}- Skill First: Always load the relevant skill before starting **complex** tasks.
 - Progressive Loading: Load resources incrementally as referenced in skills
 - Output Files: Final deliverables must be in `/mnt/user-data/outputs`
@@ -340,6 +341,7 @@ def apply_prompt_template(subagent_enabled: bool = False) -> str:
     # Add subagent reminder to critical_reminders if enabled
     subagent_reminder = (
         "- **Orchestrator Mode**: You are a task orchestrator - decompose complex tasks into parallel sub-tasks and launch multiple subagents simultaneously. Synthesize results, don't execute directly.\n"
+        "- **Citations when synthesizing**: When you synthesize subagent results that used web search or cite sources, you MUST include a consolidated `<citations>` block (JSONL format) and use [Title](url) markdown links in your response so citations display correctly.\n"
         if subagent_enabled
         else ""
     )