chore(KB-205): increase Vitest maxWorkers default from 8 to 16

- Update default VITEST_MAX_WORKERS from 8 to 16 in all packages
- Affects CLI, Core, Dashboard, and Engine vitest configs
- Improves test execution parallelism on multi-core machines
- Environment variable override still supported via VITEST_MAX_WORKERS

packages/core/vitest.config.ts

@@ -1,6 +1,6 @@
 import { defineConfig } from "vitest/config";
 
-const maxWorkers = Number.parseInt(process.env.VITEST_MAX_WORKERS ?? "8", 10);
+const maxWorkers = Number.parseInt(process.env.VITEST_MAX_WORKERS ?? "16", 10);
 
 export default defineConfig({
   test: {