feat(FN-4686): complete Step 7 — docs and changeset delivery

Fusion-Task-Id: FN-4686
Fusion-Task-Lineage: a1e58bd8-fd94-471a-b164-c396c5512fea

packages/engine/src/__tests__/worktree-acquisition-backend.test.ts

@@ -99,7 +99,7 @@ describe("acquireTaskWorktree backend wiring", () => {
   });
 
   it("throws worktrunk_operation_failed and preserves stderr", async () => {
-    execMock.mockRejectedValue({ stderr: "worktrunk exploded", code: 17 });
+    execMock.mockRejectedValue({ stderr: "worktrunk exploded", status: 17 });
 
     await expect(
       acquireTaskWorktree({

packages/engine/src/__tests__/worktree-backend.test.ts

@@ -117,7 +117,7 @@ describe("WorktrunkWorktreeBackend", () => {
   });
 
   it("throws operation failed with stderr/exitCode", async () => {
-    execMock.mockRejectedValue({ stderr: "bad news", code: 7 });
+    execMock.mockRejectedValue({ stderr: "bad news", status: 7 });
     const backend = new WorktrunkWorktreeBackend({ binaryPath: "worktrunk" });
 
     await expect(

packages/engine/src/worktree-backend.ts

@@ -133,12 +133,14 @@ export class WorktrunkWorktreeBackend implements WorktreeBackend {
         error && typeof error === "object" && "stderr" in error
           ? String((error as { stderr?: unknown }).stderr ?? "")
           : "";
-      const exitCode =
-        error && typeof error === "object" && "code" in error
-          ? (typeof (error as { code?: unknown }).code === "number"
-              ? (error as { code: number }).code
-              : null)
-          : null;
+      const execError = error && typeof error === "object" ? (error as Record<string, unknown>) : null;
+      const exitCode = execError
+        ? typeof execError.status === "number"
+          ? execError.status
+          : typeof execError.code === "number"
+            ? execError.code
+            : null
+        : null;
       this.deps.logger?.warn?.(
         `[worktree-backend] worktrunk create failed: ${stderr || String(error)}`,
       );