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>

vitest.config.ts

@@ -0,0 +1,16 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    // Use projects instead of deprecated workspace
+    projects: [
+      'libs/utils',
+      'libs/platform',
+      'libs/prompts',
+      'libs/model-resolver',
+      'libs/dependency-resolver',
+      'libs/git-utils',
+      'apps/server',
+    ],
+  },
+});