POC scaffold: extension manifest + tooling config (Feature #2)

Manifest declares the cowriting.showClineSdkInfo command; esbuild + vitest +
tsc tooling. @cline/sdk@0.0.46 as a runtime dependency. Per issue #2 (epic #1).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

vitest.config.ts

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