fix: support fork PRs by fetching via refs/pull/N/head

src/github/api/queries/github.ts

@@ -12,6 +12,7 @@ export const PR_QUERY = `
         baseRefName
         headRefName
         headRefOid
+        isCrossRepository
         createdAt
         updatedAt
         lastEditedAt

src/github/operations/branch.ts

@@ -168,7 +168,20 @@ export async function setupBranch(
 
       // 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=${fetchDepth}`, branchName]);
+      // For fork PRs, the branch doesn't exist on origin - use refs/pull/N/head instead
+      if (prData.isCrossRepository) {
+        console.log(
+          `Fork PR detected, fetching via refs/pull/${entityNumber}/head`,
+        );
+        execGit([
+          "fetch",
+          "origin",
+          `--depth=${fetchDepth}`,
+          `pull/${entityNumber}/head:${branchName}`,
+        ]);
+      } else {
+        execGit(["fetch", "origin", `--depth=${fetchDepth}`, branchName]);
+      }
       execGit(["checkout", branchName, "--"]);
 
       console.log(`Successfully checked out PR branch for PR #${entityNumber}`);

src/github/types.ts

@@ -57,6 +57,7 @@ export type GitHubPullRequest = {
   baseRefName: string;
   headRefName: string;
   headRefOid: string;
+  isCrossRepository: boolean;
   createdAt: string;
   updatedAt?: string;
   lastEditedAt?: string;

test/data-formatter.test.ts

@@ -24,6 +24,7 @@ describe("formatContext", () => {
       baseRefName: "main",
       headRefName: "feature/test",
       headRefOid: "abc123",
+      isCrossRepository: false,
       createdAt: "2023-01-01T00:00:00Z",
       additions: 50,
       deletions: 30,