feat(KB-648): enable parallel test execution and optimize test performance

- Enable parallel file execution in vitest configs for core, engine, CLI, and dashboard packages
- Optimize backup tests with fake timers for faster, deterministic execution
- Fix SettingsModal temporal dead zone by moving declaration before useCallback
- Fix engine tests with missing git branch delete mock and correct branch names
- Fix git worktree list mock and mission store mock in CLI tests
- Add changeset documenting test optimization patterns
- Update AGENTS.md with test optimization best practices

packages/cli/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"],