Point the file-length gate at the renamed TODO file

The failure message named refactor_claude_todo.md, which the markdown
namespace migration renamed to TODO-file-length-250.md. A gate telling
you to read a file that no longer exists is worse than one that says
nothing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A47aX5DJwmA3QFn3pkEWi7

file_length/check.py

@@ -175,7 +175,7 @@ def main() -> int:
     for path, lines in sorted(violations, key=lambda item: -item[1]):
         over = lines - MAX_LINES
         print(f"  {path}: {lines} lines (over by {over})", file=sys.stderr)
-    print("\nSplit them, or see refactor_claude_todo.md in this repo.", file=sys.stderr)
+    print("\nSplit them, or see TODO-file-length-250.md in this repo.", file=sys.stderr)
     return 1