feat(KB-648): enable parallel test execution across packages

- Optimize backup tests with fake timers and deterministic timestamps
- Enable Vitest fileParallelism in core, engine, cli, and dashboard packages
- Fix SettingsModal scope initialization regression covered by merge resolution
- Update affected dashboard and engine tests for the new parallel-friendly behavior
- Add a changeset and document test optimization patterns in AGENTS.md

packages/engine/src/executor.test.ts

@@ -1149,6 +1149,10 @@ describe("TaskExecutor dependency-based worktree creation", () => {
     await executor.execute(makeTask({ id: "FN-065" }));
 
     // After 3 retry attempts, should fail with combined error message
+    expect(store.updateTask).toHaveBeenCalledWith("FN-065", {
+      status: "failed",
+      error: expect.stringContaining("Worktree conflict"),
+    });
     expect(store.updateTask).toHaveBeenCalledWith("FN-065", {
       status: "failed",
       error: expect.stringContaining("automatic cleanup failed"),