PAT auth fallback to regular PRs no longer works / unclear behavior in private-preview-disabled repos

First of all, thanks for building `gh-stack` — I've been using it heavily and it has been a huge productivity improvement.

My repository is not yet enrolled in the Stacked PRs private preview, so I'm currently on the waitlist. In the meantime, I've been using `gh stack` primarily for its ability to manage and create regular PRs for a stack of branches. This workflow has been working great so far.

After upgrading from v0.0.5 to v0.0.6, that workflow stopped working because my GitHub CLI is authenticated via a Personal Access Token.

## Environment

- gh-stack 0.0.5 → worked
- gh-stack 0.0.6 → fails
- Authentication: GitHub CLI authenticated with a Personal Access Token
- Repository not enrolled in the Stacked PRs private preview

## Behavior in 0.0.5

`gh stack submit --open` successfully created and synchronized regular PRs after confirming:

```text
⚠ Stacked PRs are not enabled for this repository
Would you still like to create regular PRs? Yes
```

Example output:

```text
✓ Created PR #4654
✓ Created PR #4655
✓ Created PR #4656
```

## Behavior in 0.0.6

The command now exits with:

```text
⚠ Personal access tokens are not supported by gh stack during private preview
Run gh auth login to authenticate with OAuth instead.
```

## Question

Is this an intentional behavior change?

The v0.0.6 release notes mention improved PAT error reporting, but from a user perspective the behavior changed from successfully creating regular PRs to failing entirely.

Should PAT-authenticated users still be able to use the regular-PR fallback workflow when a repository is not enrolled in the Stacked PRs preview, or is OAuth now required for all `gh stack` operations?

Reproduce `gh stack submit --open` with a PAT in a repository without the Stacked PRs preview, comparing v0.0.5 and v0.0.6 behavior. Trace the authentication check and regular-PR fallback entry point introduced in v0.0.6. Done means the intended PAT behavior is established and the fallback either works as before or its OAuth requirement is clearly documented and tested.

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