`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)

從 `gh stack sync` 命令開始，檢查它如何驗證目前的分支，以及堆疊儲存庫使用的 `.git/gh-stack` 狀態。從一個不屬於任何堆疊的分支執行該命令；完成的標準是同步可以在那裡繼續進行，包括合併本機分支後的清理。

由索引模型根據 Issue 內容生成。