feat(sync): add dry-run summary output with change totals

[sonwr]

Summary

• add a dry-run summary block for sync --dry-run that reports create/update/delete totals
• print per-tool breakdown and aggregate totals to improve operator readability
• update README with a concrete dry-run output example
• add a node:test regression test for dry-run summary output

Testing

• npm test
  • pass: dry-run summary reports created/updated/deleted files with totals

Notes

• keeps existing sync behavior unchanged for non-dry-run paths
• dry-run mode remains non-destructive (no write/remove side effects)