After merging all PRs in a stack and pruning the branches locally, there is no way to close a stack locally

[ice1080]

Steps:

• create a stack with only one branch
• submit the stack
• merge the PR
• sync the stack, pruning the branch

At this point, there is no way to remove the stack from local. Running github stack view lists it still, but attempting to switch to it throws this error:

✗ failed to checkout branch <branch-name>: failed to run git: error: pathspec '<branch-name>' did not match any file(s) known to git

Because it does not list a stack number in this view, there is also no way to run unstack.

There's a chance this is only an issue when there is only one branch in the stack, but it should still work regardless (for cases where it was determined no other work is needed, or later branches were removed so the stack only had one remaining).

Request - after syncing a stack that has been merged (even if only one PR in it), the stack should be deleted locally too.