* Add PR vouch workflow and trust-status labeling

- Add `.github/workflows/pr-vouch.yml` to label PRs as trusted, unvouched, or denounced
- Add `.github/VOUCHED.td` trust-list file for external contributors
- Document automatic `vouch:*` labels and contributor expectations in `CONTRIBUTING.md`

* Add vouch entries for trusted GitHub contributors

- Append 16 GitHub accounts to `.github/VOUCHED.td`
- Update trusted-voucher list used for PR labeling workflows