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/api/queries/github.ts

@@ -12,6 +12,13 @@ export const PR_QUERY = `
         baseRefName
         headRefName
         headRefOid
+        isCrossRepository
+        headRepository {
+          owner {
+            login
+          }
+          name
+        }
         createdAt
         updatedAt
         lastEditedAt

src/github/operations/branch.ts

@@ -166,9 +166,23 @@ export async function setupBranch(
       // Validate branch names before use to prevent command injection
       validateBranchName(branchName);
 
-      // Execute git commands to checkout PR branch (dynamic depth based on PR size)
-      // Using execFileSync instead of shell template literals for security
-      execGit(["fetch", "origin", `--depth=${fetchDepth}`, branchName]);
+      // For cross-repository (fork) PRs, fetch via the pull ref since the
+      // branch only exists on the fork's remote, not on origin.
+      if (prData.isCrossRepository) {
+        console.log(
+          `PR #${entityNumber} is from a fork, fetching via refs/pull/${entityNumber}/head...`,
+        );
+        execGit([
+          "fetch",
+          "origin",
+          `--depth=${fetchDepth}`,
+          `pull/${entityNumber}/head:${branchName}`,
+        ]);
+      } else {
+        // Execute git commands to checkout PR branch (dynamic depth based on PR size)
+        // Using execFileSync instead of shell template literals for security
+        execGit(["fetch", "origin", `--depth=${fetchDepth}`, branchName]);
+      }
       execGit(["checkout", branchName, "--"]);
 
       console.log(`Successfully checked out PR branch for PR #${entityNumber}`);

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;