When a PR originates from a fork, `git fetch origin <branch>` fails
because the branch only exists on the fork's remote.
Fix: detect cross-repository PRs via the `isCrossRepository` GraphQL
field and fetch using `pull/<number>/head:<branch>` refspec instead,
which is the standard GitHub mechanism for accessing fork PR branches.
Changes:
- Add `isCrossRepository` and `headRepository` to PR GraphQL query
- Add corresponding fields to GitHubPullRequest type
- Branch checkout uses pull ref for fork PRs
- Update test fixtures with new fields
Co-authored-by: User <user@example.com>
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