Fix SystemExit traceback during atexit cleanup on Ctrl+C

The browser_tool signal handler calls sys.exit(130) which raises
SystemExit. When this fires during terminal_tool's atexit cleanup
(specifically during _cleanup_thread.join()), it produces an unhandled
traceback. Wrapping the join in a try/except suppresses the race
without changing shutdown behavior.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tools/terminal_tool.py

@@ -617,7 +617,10 @@ def _stop_cleanup_thread():
     global _cleanup_running
     _cleanup_running = False
     if _cleanup_thread is not None:
-        _cleanup_thread.join(timeout=5)
+        try:
+            _cleanup_thread.join(timeout=5)
+        except (SystemExit, KeyboardInterrupt):
+            pass
 
 
 def get_active_environments_info() -> Dict[str, Any]: