Should we support retag operations?

[bcfre]

Description

The current implementation assumes that if a pending manifest file exists in the image repository, the upload has already been completed.
However, this assumption introduces two potential issues:

1. Inability to rename model artifacts efficiently
Users cannot rename model artifacts using the build --output-remote command.
The only available workflow is: delete the existing artifact → rebuild → upload → rename.
This adds an unnecessary re-upload step.

2. Silent failure when tags differ
If an existing manifest references tag-a and the user builds with tag-b,
the logic will neither retag the artifact nor throw an error.
As a result, the build step appears to succeed, but pulling using tag-b will fail because the tag does not actually exist in the repository.

Additional Context

Since reproducible builds are not currently supported[#243 ], this issue is likely to occur only in scenarios with high concurrency.

It's possible I might be missing some background context here, so please let me know if my understanding is incorrect.

https://github.com/modelpack/modctl/blob/main/pkg/backend/build/remote.go#L127-L137

        exist, err := ro.remote.Manifests().Exists(ctx, desc)
	if err != nil {
		hooks.OnError(digest, err)
		return ocispec.Descriptor{}, fmt.Errorf("failed to check if blob exists: %w", err)
	}
	if exist {
		hooks.OnComplete(digest, desc)
		return desc, nil
	}

[chlins]

@bcfre Thank you for highlighting these issues. Regarding your two concerns:

1. We currently have the modctl tag command, but it only supports local builds—not remote ones. We may need to enhance this command to cover remote builds as well.
2. We should also ensure that we verify the existence of the tag on the artifact.

Please let me know if I have understood your concerns correctly.