docs: document sync test refactoring findings and patterns

- Add comprehensive test pattern guidance to tests/sync/mod.rs
- Explain when to use run_sync_test() vs manual setup
- Document helper scope and architectural limitations

Key findings:
- historic_sync.rs: 4 tests refactored, 143 lines removed (50% reduction)
- live_sync/discovery/tag_variations: Manual setup required due to
  architectural incompatibilities (timing, multi-relay, assertions)
- Helper works for batch historic verification, not real-time scenarios

Detailed summary available in work/sync-test-refactor-summary.md

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