GitLab: delete after merge

[abhinav]

Discussed in #823

^{Originally posted by maritz August 26, 2025}
Currently, it looks like git-spice disables the MR (GitLab CR) setting for a source branch to be deleted after merge.

And it also doesn't automatically delete them when they are locally deleted.

Is this intentional? Am I doing something wrong? This would mean that we accrue a lot of stale branches over time, if not manually pruned.