Manage Atom feeds in a persistent git repository

Fix links.json to support multiple source articles per URL and resolve url_mapping.json empty file issue

- Changed links.json structure to map URLs to lists of atom IDs instead of single IDs
- Added proper deduplication to avoid storing duplicate entry IDs for same URL
- Fixed url_mapping.json by converting HttpUrl objects to strings for proper comparison
- Added debug output to show registered URLs count for troubleshooting
- Updated filtering logic to handle new list-based link structure

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

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

Changed files
+17 -11
src
thicket
cli
commands