feat: initial Go/JS/Rust Origin check-in client SDKs

Standalone clients that resolve a machine unique ID (OS machine ID first,
falling back to a generated UUID persisted to disk), detect hostname/IP/
container/orchestrator, and register+re-ping origin.warky.dev's public
service-checkin endpoint on a daily interval. Ported from icy2's internal
origincheckin package but extended to the full check-in contract and made
dependency-light for external reuse.

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

go/machineid_linux.go

@@ -0,0 +1,24 @@
+//go:build linux
+
+package originclient
+
+import (
+	"os"
+	"strings"
+)
+
+// machineID reads the systemd/dbus machine ID, which is stable across
+// reboots and unique per Linux install (but not per container, since it is
+// typically inherited from the image unless explicitly reset).
+func machineID() (string, error) {
+	for _, path := range []string{"/etc/machine-id", "/var/lib/dbus/machine-id"} {
+		data, err := os.ReadFile(path)
+		if err != nil {
+			continue
+		}
+		if id := strings.TrimSpace(string(data)); id != "" {
+			return id, nil
+		}
+	}
+	return "", errMachineIDUnavailable
+}