refactor: use Vitest projects config instead of deprecated workspace

- Add root vitest.config.ts with projects array (replaces deprecated workspace)
- Add name property to each project's vitest.config.ts for filtering
- Update package.json test scripts to use vitest projects
- Add vitest to root devDependencies

This addresses the Vitest warning about multiple configs impacting
performance by running all projects in a single Vitest process.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

libs/prompts/vitest.config.ts

@@ -2,6 +2,7 @@ import { defineConfig } from 'vitest/config';
 
 export default defineConfig({
   test: {
+    name: 'prompts',
     globals: true,
     environment: 'node',
     include: ['tests/**/*.test.ts'],