Refactor detect_changes to use common-ci-configuration template

- Replace 140+ lines of custom change detection with .haf_app_detect_changes
  template from common-ci-configuration
- Move cache lookup logic to sync job where it belongs
- Sync job now gracefully falls back to full sync if no cache available
  in AUTO_SKIP_SYNC mode (QUICK_TEST still requires explicit cache)
- Export SYNC_CACHE_KEY via dotenv artifact for e2e job
- Clean up unused AUTO_CACHE_KEY variable

Net reduction: ~80 lines of CI config