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/examples/testclient/main.go

@@ -0,0 +1,98 @@
+// Command testclient is a runnable smoke test for the Go originclient SDK.
+//
+// Without ORIGIN_SERVICE_KEY set, it spins up a local mock check-in server
+// and exercises the client against it — no network, no credentials needed.
+// With ORIGIN_SERVICE_KEY set, it performs a real check-in against
+// ORIGIN_URL (or Origin's public endpoint) so the SDK can be smoke-tested
+// end-to-end against a live server.
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"flag"
+	"fmt"
+	"log"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"os/signal"
+	"strconv"
+	"syscall"
+
+	originclient "git.warky.dev/wdevs/origin_client/go"
+)
+
+func main() {
+	watch := flag.Bool("watch", false, "run continuously via Run() instead of a single check-in")
+	flag.Parse()
+
+	cfg := originclient.Config{
+		URL:         os.Getenv("ORIGIN_URL"),
+		ServiceKey:  os.Getenv("ORIGIN_SERVICE_KEY"),
+		Type:        envOr("ORIGIN_TYPE", "icy2-testclient"),
+		Version:     envOr("ORIGIN_VERSION", "0.0.0-test"),
+		Name:        envOr("ORIGIN_NAME", "icy2-testclient-local"),
+		Site:        os.Getenv("ORIGIN_SITE"),
+		Environment: os.Getenv("ORIGIN_ENVIRONMENT"),
+		Logger:      originclient.LoggerFunc(func(msg string, err error) { log.Printf("[warn] %s: %v", msg, err) }),
+	}
+
+	if cfg.ServiceKey == "" {
+		log.Println("ORIGIN_SERVICE_KEY not set — using a local mock check-in server")
+		server := httptest.NewServer(http.HandlerFunc(mockHandler))
+		defer server.Close()
+		cfg.URL = server.URL
+		cfg.ServiceKey = "test-key"
+		cfg.InstallID = "test-install-id"
+	}
+
+	if h := os.Getenv("ORIGIN_INTERVAL_HOURS"); h != "" {
+		if n, err := strconv.Atoi(h); err == nil {
+			cfg.IntervalHours = n
+		}
+	}
+
+	client, err := originclient.New(cfg)
+	if err != nil {
+		log.Fatalf("originclient.New: %v", err)
+	}
+
+	if *watch {
+		ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
+		defer stop()
+		log.Println("watching — Ctrl-C to stop")
+		client.Run(ctx)
+		return
+	}
+
+	resp, err := client.CheckinOnce(context.Background())
+	if err != nil {
+		log.Fatalf("checkin failed: %v", err)
+	}
+	out, _ := json.MarshalIndent(resp, "", "  ")
+	fmt.Println(string(out))
+}
+
+func mockHandler(w http.ResponseWriter, r *http.Request) {
+	var payload originclient.Payload
+	_ = json.NewDecoder(r.Body).Decode(&payload)
+	log.Printf("mock server received check-in: type=%s name=%s install_id=%s key=%s",
+		payload.Type, payload.Name, payload.UniqueInstallID, r.Header.Get("X-Origin-Service-Key"))
+
+	w.Header().Set("Content-Type", "application/json")
+	_ = json.NewEncoder(w).Encode(originclient.Response{
+		Success:           true,
+		ServiceInstanceID: 1,
+		UniqueInstallID:   payload.UniqueInstallID,
+		Status:            "provisioning",
+		PingedAt:          "2026-08-01T00:00:00Z",
+	})
+}
+
+func envOr(key, def string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return def
+}