`gh stack sync` can't be run anywhere in the repository

If you're on a repository that uses Stacked PRs, but you're not on a branch that's currently stacked, you see:

```
% gh stack sync
✗ current branch "fix/number" is not part of a stack
Checkout an existing stack using `gh stack checkout` or create a new stack using `gh stack init`
```

This means that if you want to leverage `.git/gh-stack` as the internal state for what branches are available, you need to now go onto a stacked branch to sync.

There should be a way to sync without necessarily being on a stacked branch (for instance, if you want to make sure that your local branch is cleaned up after it's been merged)

Start at the `gh stack sync` command and inspect how it validates the current branch, along with the `.git/gh-stack` state used by stacked repositories. Run the command from a branch that is not part of a stack; done means synchronization can proceed there, including cleanup after a merged local branch.

索引モデルが issue の本文から書いたものです。