refactor: normalise skipped check summaries

- Add IDENTITY_CHECK_SUMMARIES const mapping check IDs to human-readable summaries
- Implement summary_for_check_id helper to centralize summary lookup
- Replace split-and-underscore patterns with consistent space-separated text
- Use helper in all Skipped CheckResult creations for uniform formatting

Fixes M-NEW-5: Snapshot tests now show consistent 'target resolved' vs 'target_resolved' formatting

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>