feat: add Vitest test infrastructure for agent and script validation (#337)

Adds Vitest + TypeScript test infrastructure for agent validation. Validates 642 agent files across 14 categories for YAML frontmatter, kebab-case naming, and directory population.

vitest.config.ts

@@ -0,0 +1,8 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    include: ["tests/**/*.test.ts"],
+    testTimeout: 30_000,
+  },
+});