- Create IdempotencyChecker class with check_exists method
- Check for metadata JSON and content Markdown files
- Return tuple of (all_exist, existing_files)
- Follow naming convention: {platform}_{content_id}_{type}.{ext}
- Add get_expected_files helper method
- Add 8 unit tests covering all scenarios
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
