`gh stack submit` fails on forks by attempting to create PRs on upstream repository (`Head sha can't be blank`)

When using `gh stack submit` on a repository cloned from a **fork**, the command attempts to query and submit the stack against the upstream parent repository instead of the fork (or specified default repository).

Since the feature branches only exist on the fork (`origin`), GitHub's GraphQL API rejects the PR creation because the branches do not exist on the target upstream repository, resulting in `Head sha can't be blank` / `Base sha can't be blank`.

Executing `gh repo set-default /` does not alter this behavior.

### Environment
- **OS:** Windows 11 (Git Bash / MINGW64)
- **`gh` version:** `gh version 2.100.0 (2026-09-03)`
- **`gh stack` extension version:** `github/gh-stack v0.1.1`

### Repository Setup
- **origin:** [https://github.com/Hakzai/pokerpulsepro-tauri.git](https://github.com/Hakzai/pokerpulsepro-tauri.git) (Fork)
- **upstream:** [https://github.com/davidelvar/pokerpulsepro-tauri.git](https://github.com/davidelvar/pokerpulsepro-tauri.git) (Parent Repo)
- `gh repo set-default` is explicitly set to `Hakzai/pokerpulsepro-tauri`.

### Steps to Reproduce
1. Clone a fork repository that has an `upstream` remote configured.
2. Create stacked branches locally and push them to `origin`:
a. `main` -> `feature/branch-1` -> `feature/branch-2`
3. Set the default repository using `gh repo set-default Hakzai/pokerpulsepro-tauri`.
4. Run `gh stack submit`.

### Expected Behavior
`gh stack submit` should resolve stacked PRs against the configured default repository (`Hakzai/pokerpulsepro-tauri`) or provide cross-repository fork support by specifying the head branch owner/ref.

### Actual Behavior
The command targets `davidelvar/pokerpulsepro-tauri` and fails with:

```log
⚠ failed to create PR for feature/add-brazil-real-currency: creating PR: GraphQL: Head sha can't be blank, Base sha can't be blank, No commits between main and feature/add-brazil-real-currency, Head ref must be a branch (createPullRequest)
```

#### Debug Output (`GH_DEBUG=1 gh stack submit`)
Relevant log excerpts showing API requests targeting the upstream repo instead of the fork:

```log
* Request to https://api.github.com/repos/davidelvar/pokerpulsepro-tauri/stacks?per_page=100&page=1
...
[git.exe push origin --force-with-lease=refs/heads/feature/add-brazil-real-currency:... refs/heads/feature/add-brazil-real-currency:refs/heads/feature/add-brazil-real-currency]
* Request to https://api.github.com/graphql
⚠ failed to create PR for feature/add-brazil-real-currency: creating PR: GraphQL: Head sha can't be blank, Base sha can't be blank, No commits between main and feature/add-brazil-real-currency, Head ref must be a branch (createPullRequest)
```

#### Git Config
Git config is pointing correctly the upstream to `davidelvar/pokerpulsepro-tauri` and the origin to the fork at `Hakzai/pokerpulsepro-tauri`.

```ini
submodule.active=.
remote.origin.url=https://github.com/Hakzai/pokerpulsepro-tauri.git
remote.origin.fetch=+refs/heads/*:refs/remotes/origin/*
remote.origin.gh-resolved=base
branch.main.remote=origin
branch.main.merge=refs/heads/main
branch.main.vscode-merge-base=origin/main
lfs.repositoryformatversion=0
remote.upstream.url=https://github.com/davidelvar/pokerpulsepro-tauri.git
remote.upstream.fetch=+refs/heads/*:refs/remotes/upstream/*
...
branch.feature/full-backup-impl.vscode-merge-base=origin/main
branch.feature/full-backup-impl.remote=origin
branch.feature/full-backup-impl.merge=refs/heads/feature/full-backup-impl
branch.feature/add-brazil-real-currency.vscode-merge-base=origin/main
branch.feature/add-brazil-real-currency.remote=origin
branch.feature/add-brazil-real-currency.merge=refs/heads/feature/add-brazil-real-currency
branch.feature/create-winners-board.vscode-merge-base=origin/main
branch.feature/create-winners-board.remote=origin
branch.feature/create-winners-board.merge=refs/heads/feature/create-winners-board
branch.bugfix/prize-tab-not-showing-champion.vscode-merge-base=origin/main
branch.bugfix/prize-tab-not-showing-champion.remote=origin
branch.bugfix/prize-tab-not-showing-champion.merge=refs/heads/bugfix/prize-tab-not-showing-champion
branch.chore/deploy-to-github-pages.vscode-merge-base=origin/main
branch.chore/deploy-to-github-pages.remote=origin
branch.chore/deploy-to-github-pages.merge=refs/heads/chore/deploy-to-github-pages
rerere.enabled=true
rerere.autoupdate=true
gh-stack.remote=origin
```

`gh stack submit` のエントリーポイントから始め、`gh-stack.remote` と設定されたデフォルトリポジトリを含むリポジトリ解決を、GraphQL の `createPullRequest` 呼び出しまで追跡します。フォークのセットアップと `GH_DEBUG=1` を使って再現します。スタックされた PR の作成がフォークを対象にするか、意図した head リポジトリを明示的にサポートし、空の SHA エラーが発生しなければ完了です。

索引モデルが issue の本文から書いたものです。