Submodules not disabled for git mirrors

Hi, I think I've hit an edge case with git mirrors and submodules disabled with agent-stack-k8s.

It seems that `git submodule foreach --recursive "git clean -ffxdq"` gets run during git mirror updates, even when we have disabled submodules (using the agent-stack-k8s noSubmodules option which just sets the corresponding env var under the hood).

This doesn't seem to cause an issue normally, I guess because the submodules weren't initialized, but we recently had a change committed by accident which added an unused submodule without a corresponding `.gitmodules` entry. This caused the above command to complain `fatal: No url found for submodule path 'xyz' in .gitmodules` which completely broke all our builds once the commit got to main :(

It seems like when submodules are disabled, the git mirror functionality should ignore submodules as well? Or have a separate option to disable submodules for mirrors that we could set.