feat: add --check-sites command for batch site validation

[mvanhorn]

Summary

Adds a --check-sites flag that batch-validates every site in data.json by testing each site's username_claimed value against the existing detection logic. Reports broken, timed out, and healthy sites with a summary table.

Why this matters

Sites break constantly: domains expire, WAFs change, pages restructure. Right now there's no way to proactively check which sites are still working. Maintainers rely on individual user reports.

Source	Evidence
#2818	False positive: cracked.sh
#2815	False positive: 1337x.to
#2808	False negative: omg.lol
#2805	False negative: Patched
#2804	False negative: GNOME VCS
#2803	False negative: BoardGameGeek

Six open FP/FN reports in recent months. A batch validation command catches these proactively.

Changes

• Added check_sites() function in sherlock.py that iterates over all sites, runs the existing sherlock() detection on each site's username_claimed user, and classifies results as OK/BROKEN/TIMEOUT
• Added --check-sites argparse flag. Changed username positional to nargs="*" with a custom validation that requires either usernames or --check-sites
• Uses the base QueryNotify class (silent) to suppress per-site output during health checks
• Exit code 1 when broken sites detected (CI-friendly)
• Updated test in test_ux.py to match the new nargs="*" behavior for the unrecognized-argument edge case

Demo

Testing

• Existing pytest suite passes (21 passed, 4 deselected)
• Manual testing with --check-sites --site GitHub --site Reddit --site Twitter --timeout 15 shows clean output
• Normal username search unaffected

The check_sites() function reuses the existing sherlock() function and SherlockFuturesSession for parallel requests, so it runs against the same detection logic users already rely on.

This contribution was developed with AI assistance (Claude Code).