Add vitest test infrastructure for the package

package.json

@@ -8,11 +8,17 @@
     "extensions": ["./extensions"],
     "prompts": ["./prompts"]
   },
+  "scripts": {
+    "test": "vitest --run"
+  },
   "peerDependencies": {
     "@earendil-works/pi-ai": "*",
     "@earendil-works/pi-agent-core": "*",
     "@earendil-works/pi-coding-agent": "*",
     "@earendil-works/pi-tui": "*",
     "typebox": "*"
+  },
+  "devDependencies": {
+    "vitest": "^3.2.4"
   }
 }

vitest.config.ts

@@ -0,0 +1,7 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    include: ["extensions/**/*.test.ts"],
+  },
+});