gh stack modify fails with untracked files

`gh stack modify` preconditions fail if the current working directory has any untracked files and fails with the following message that seems to imply that untracked changes shouldn't be a problem, as there is nothing to stash or commit.

```
✗ uncommitted changes in working tree
Commit or stash your changes before running modify
```

Code that performs the check: https://github.com/github/gh-stack/blob/5ed5693ed6fcfa34228312abb09553e2580b6670/internal/git/gitops.go#L576

Perhaps this should ignore any lines starting with `?? `?

Start in internal/git/gitops.go around the precondition check at line 576 and reproduce the failure with an untracked file in the working directory. Inspect how Git status output is classified, then verify that untracked files no longer block `gh stack modify` while genuinely uncommitted changes still do.

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