fix: handle fork PRs by fetching via refs/pull/N/head (#962) (#963)

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>

test/pull-request-target.test.ts

@@ -17,6 +17,8 @@ describe("pull_request_target event support", () => {
       baseRefName: "main",
       headRefName: "feature-branch",
       headRefOid: "abc123",
+      isCrossRepository: false,
+      headRepository: { owner: { login: "testowner" }, name: "testrepo" },
       commits: {
         totalCount: 2,
         nodes: [