gh stack sync --prune for a completed stack deletes the branch but leaves stack in gh stack checkout metadata

[rwedoff]

gh stack sync --prune for a completed stack deletes the branch but leaves stack in gh stack checkout metadata.

Repro steps:

1. Create a stack with 1 branch.
2. Submit PR
3. Complete PR
4. Run gh stack sync --prune
5. Branch is synced and deleted
6. Run gh stack checkout
7. Observe

Expected: Stack should be deleted
Actual: Stack is not deleted, and cannot be selected because the branch no longer exists.
Workaround: Create a branch with the same name as one that doesn't exist. Run gh stack untrack, delete branch.