Migrate the Docker image to grate-devs/grate

## Context

Follow-up to #796. The release image is currently published as `erikbra/grate`, while the source repository and project ownership have moved to `grate-devs`.

Docker Hub does not redirect image references between namespaces, so this needs an explicit compatibility plan.

## Scope

- Claim or create the `grate-devs` Docker Hub organization namespace and a public `grate-devs/grate` repository.
- Configure organization-owned or appropriately scoped CI credentials.
- Copy all historical tags from `erikbra/grate` to `grate-devs/grate`, preserving multi-architecture manifests and verifying digests.
- Change release automation so `grate-devs/grate:` is the canonical image.
- Dual-publish identical version tags to `erikbra/grate` during an agreed transition period.
- Ensure `latest` is updated only for stable releases and never for prereleases.
- Mark the old Docker Hub repository as moved, linking to the new image.
- Update local Docker build tooling and tests to use the new canonical name.

## Acceptance criteria

- [ ] Historical tags required by users are available under `grate-devs/grate`.
- [ ] Matching copied tags have verified manifest digests/platforms.
- [ ] A release publishes the version tag to the new namespace.
- [ ] A stable release updates `latest`; a prerelease does not.
- [ ] The compatibility behavior for `erikbra/grate` is documented and tested.
- [ ] CI credentials are not tied unnecessarily to a single maintainer.
- [ ] Documentation changes are coordinated with the distribution documentation sweep.

## References

- #796
- Docker image migration: https://docs.docker.com/docker-hub/repos/manage/hub-images/bulk-migrate/