Add runnable smoke-test clients for each SDK

Each package gets a small example program that exercises the real
client: with no ORIGIN_SERVICE_KEY set it runs against a local mock
check-in server, otherwise it checks in against ORIGIN_URL/Origin's
public endpoint. Wired into CI (mock mode) as a regression check, and
documented per README.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

js/package.json

@@ -21,7 +21,8 @@
   "scripts": {
     "build": "tsc -p tsconfig.json",
     "test": "vitest run",
-    "typecheck": "tsc --noEmit"
+    "typecheck": "tsc --noEmit",
+    "example": "npm run build && node examples/testclient.mjs"
   },
   "devDependencies": {
     "@types/node": "^22.10.0",