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>

src/github/types.ts

@@ -57,6 +57,13 @@ export type GitHubPullRequest = {
   baseRefName: string;
   headRefName: string;
   headRefOid: string;
+  isCrossRepository: boolean;
+  headRepository: {
+    owner: {
+      login: string;
+    };
+    name: string;
+  } | null;
   createdAt: string;
   updatedAt?: string;
   lastEditedAt?: string;