Unify links, references, and url_mapping JSON files into single structure
- Consolidate references.json, url_mapping.json, and links.json into unified links.json
- Eliminate duplication by storing each URL only once with metadata flags
- Add is_tracked_post flag and target_username to distinguish internal/external links
- Update all commands (links, index, threads, info) to use unified structure
- Maintain all existing functionality while reducing file I/O and complexity
- Update documentation and help text to reflect changes
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Co-Authored-By: Claude <noreply@anthropic.com>