Incorrect "PR #5 remains open" remains open after git stack modify for a PR that is already closed

1. Use `gs submit` to create stack with depth 2. Let's say branch names `layer1` and `layer2`
2. I close the PR for `layer2` in the GitHub UI
3. I want to remove the stack locally, so I use `git stack modify`, press `x` to delete the layer, and then `Control+S` to save.

I get the following output
```
$ gs modify
✓ Dropped layer2 from stack

✓ Stack modified successfully
Dropped: layer 2(PR #53155 remains open — close with `gh pr close 53155`)

Run `gh stack submit` to push your changes and update the stack of PRs on GitHub
```

Now `gh` cli can clearly tell the PR is already closed, because when I try to close it:
```
$ gh pr close 53155
! Pull request repo#53155 (PR Title) is already closed
```

I think a fix will be doing a resync on the state from GitHub before showing this message.

Start with the `git stack modify` save flow, including the `gs modify` output that reports a dropped layer as still open. Check how the GitHub PR state is obtained before that message; done when a PR already closed in GitHub is not reported as open after the layer is deleted.

索引モデルが issue の本文から書いたものです。