Replace the Jenkinsfile with .gitea/workflows/ci.yml. The CI image is
built from the Dockerfile and pushed to the Gitea container registry,
rebuilt only when the Dockerfile or pre-commit config changes. The
aarch64 release build now runs natively on an arm64 runner instead of
cross-compiling, so the cross toolchain and qemu are dropped from the
image. Artifacts still go to MinIO (via mc, skipped gracefully if
credentials are not configured); ccache uses actions/cache instead of a
host volume.
