CI: switch from CircleCI to GitHub Actions

[reinkrul]

The unit-test / coverage pipeline (.circleci/config.yml) is the last thing on CircleCI — everything else already runs on GitHub Actions. Consolidate.

Scope

• Replace .circleci/config.yml with a GHA workflow running go test ./... -race on PRs and master pushes.
• Upload coverage to Qlty via qltysh/qlty-action/coverage (supports OIDC, no token secret needed).
• Delete .circleci/config.yml; update CircleCI build badge in README.rst; disable the CircleCI project once green.

Reference

• nuts-admin/.github/workflows/go-test.yaml for a minimal Go-on-GHA template.
• Single-runner port is fine; the current 8-way circleci tests split and coverage merge drop out. Shard via matrix later only if needed.