feat(KB-648): optimize test execution speed across all packages

- Enable parallel file execution in core, engine, CLI, and dashboard packages via vitest.config.ts

- Optimize backup tests using fake timers instead of real setTimeout delays

- Add test optimization patterns to AGENTS.md documentation (fake timers, parallel execution)

- Add changeset documenting the test speed improvements

- Include test fixes for SettingsModal, engine tests, and git worktree mocks from KB-637

packages/engine/vitest.config.ts

@@ -6,7 +6,7 @@ export default defineConfig({
   test: {
     include: ["src/**/*.test.ts"],
     maxWorkers,
-    fileParallelism: false,
+    fileParallelism: true,
     coverage: {
       enabled: false,
       reporter: ["text", "html", "json"],