Cross-repo push-to-pull-request-branch doesn't have access to correct repo contents

[dsyme]

In this run the push-to-pull-request-branch is not able to easily work on the right pull request.

Note it's a schedule job so the repo context Git history is definitey not set up to allow working on the pull request - the Git history for checkout: will only have main

checkout:
  - repository: githubnext/gh-aw-side-repo
    token: ${{ secrets.GH_AW_SIDE_REPO_PAT }}

https://github.com/github/gh-aw/actions/runs/22577183185/job/65399412001

We should add options to checkout: to

1. fetch all branches (fetch: ["*"] ), or
2. fetch all branches for all open pull requests (fetch: ["pulls/open/*"] - a special GH-AW way of saying all open PRs)
3. fetch specific branches by pattern e.g. fetch: ["main", "feature/*"]
4. fetch specific branches e.g. fetch: ["main", "feature/auto-perf"]

We should also extend the following prompting to be clearer about (1) what fetch-depth the checkouts have and (2) what branches are available and (3) say that if an existing branch you want to work from isn't available and it's a private repository then you won't be able to access the Git history of that branch properly, and you should probably exit with an error.

- **checkouts**: The following repositories have been checked out and are available in the workspace:
  - `$GITHUB_WORKSPACE` → `githubnext/gh-aw-side-repo` (cwd)