feat: add AT Proto testing, force-sync mode, and improved sync logic
- Expand README with clearer config steps and new AT Proto testing section
- Add `test-atproto`, `check`, `validate`, and `sync:force` scripts
- Convert project to ESM (`"type": "module"`)
- Improve Tangled record schema handling and error reporting
- Add force-sync flag and skip logic for existing repos
- Enhance logging, startup info, and repo processing flow
- Update tsconfig to NodeNext module resolution
- Minor dependency and package-lock adjustments