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/gateway/path_utils.py

@@ -0,0 +1,44 @@
+"""Shared path resolution for thread virtual paths (e.g. mnt/user-data/outputs/...)."""
+
+import os
+from pathlib import Path
+
+from fastapi import HTTPException
+
+from src.agents.middlewares.thread_data_middleware import THREAD_DATA_BASE_DIR
+
+# Virtual path prefix used in sandbox environments (without leading slash for URL path matching)
+VIRTUAL_PATH_PREFIX = "mnt/user-data"
+
+
+def resolve_thread_virtual_path(thread_id: str, virtual_path: str) -> Path:
+    """Resolve a virtual path to the actual filesystem path under thread user-data.
+
+    Args:
+        thread_id: The thread ID.
+        virtual_path: The virtual path (e.g., mnt/user-data/outputs/file.txt).
+                      Leading slashes are stripped.
+
+    Returns:
+        The resolved filesystem path.
+
+    Raises:
+        HTTPException: If the path is invalid or outside allowed directories.
+    """
+    virtual_path = virtual_path.lstrip("/")
+    if not virtual_path.startswith(VIRTUAL_PATH_PREFIX):
+        raise HTTPException(status_code=400, detail=f"Path must start with /{VIRTUAL_PATH_PREFIX}")
+    relative_path = virtual_path[len(VIRTUAL_PATH_PREFIX) :].lstrip("/")
+
+    base_dir = Path(os.getcwd()) / THREAD_DATA_BASE_DIR / thread_id / "user-data"
+    actual_path = base_dir / relative_path
+
+    try:
+        actual_path = actual_path.resolve()
+        base_resolved = base_dir.resolve()
+        if not str(actual_path).startswith(str(base_resolved)):
+            raise HTTPException(status_code=403, detail="Access denied: path traversal detected")
+    except (ValueError, RuntimeError):
+        raise HTTPException(status_code=400, detail="Invalid path")
+
+    return actual_path