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>

go/README.md

@@ -43,3 +43,14 @@ check-in and observe the error/response.
 `Hostname()`, `OutboundIP()`, `ContainerID()`, `Orchestrator()`, and
 `ResolveMachineID()` are exported standalone for callers that want the
 detection logic without the HTTP client.
+
+## Manual smoke test
+
+```bash
+go run ./examples/testclient           # single check-in, local mock server
+go run ./examples/testclient -watch    # loops via Run() until Ctrl-C
+```
+
+Set `ORIGIN_SERVICE_KEY` (and optionally `ORIGIN_URL`, `ORIGIN_TYPE`,
+`ORIGIN_VERSION`, `ORIGIN_NAME`, `ORIGIN_SITE`, `ORIGIN_ENVIRONMENT`,
+`ORIGIN_INTERVAL_HOURS`) to check in against a real endpoint instead.