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

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.

Reproduce el problema con un stack de una sola rama ejecutando `gh stack sync --prune` después de que se complete su PR y, a continuación, ejecuta `gh stack checkout`. Traza la ruta de prune de sync y el manejo de los metadatos de checkout; se considera terminado cuando el stack completado se elimina de los metadatos y deja de mostrarse como un stack seleccionable no válido.

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