Reordering a stack can cause PRs to be incorrectly marked as merged

## Problem

After reordering a published stack, I ran:

```bash
gh stack modify
gh stack push
gh stack submit
```

`gh stack push` updated the branch ancestry without updating the existing PR bases. Some PRs then targeted descendant branches that already contained their commits.

GitHub saw empty diffs and automatically marked those PRs as merged, although their commits were never merged into `main`. The PRs could not be reopened and had to be recreated with new branch names.

## Version

`gh stack version 0.0.8`

## Expected behavior

I expected `gh stack push` either
- to update the remote stack to match the reordered local stack,
- or to fail and tell me to use `gh stack submit` for this operation.

Start with the implementation behind `gh stack push`, then trace how reordered stack ancestry and existing PR bases are handled. Reproduce the `gh stack modify`, `gh stack push`, and `gh stack submit` sequence from the report; done means push safely synchronizes the remote stack or clearly directs users to submit, without PRs being incorrectly marked as merged.

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