Add Linux systemd support and --install-service/--remove-service flags

The service package now has a Linux implementation alongside the Windows
one: systemd unit install/remove (/etc/systemd/system), readiness
notification (READY=1 via go-systemd) sent only after the listeners are
bound, a 30s watchdog, and STOPPING on shutdown. Listeners are pre-bound
so port conflicts fail fast and the readiness signal is truthful. The
notify calls are no-ops without NOTIFY_SOCKET (containers, shells) and
on non-Linux builds. --install-service/--remove-service work on both
platforms; the 'service install|remove' subcommand remains as an alias.

internal/service/service_linux_test.go

@@ -0,0 +1,23 @@
+//go:build linux
+
+package service
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestRenderUnit(t *testing.T) {
+	unit := renderUnit("/usr/local/bin/gpu-turnstile", "/etc/gpu-turnstile.env")
+	for _, want := range []string{
+		"Type=notify",
+		"WatchdogSec=30s",
+		`ExecStart="/usr/local/bin/gpu-turnstile" -config "/etc/gpu-turnstile.env"`,
+		"Restart=on-failure",
+		"WantedBy=multi-user.target",
+	} {
+		if !strings.Contains(unit, want) {
+			t.Fatalf("unit missing %q:\n%s", want, unit)
+		}
+	}
+}