feat: add signal-bound server supervisor command

server/cmd/game-server-supervisor/main_test.go

@@ -0,0 +1,13 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/cosmic-clash/cosmic-clash/server/supervisor"
+)
+
+func TestSupervisorCommandUsesTheSameProductionGraceDefault(t *testing.T) {
+	if supervisor.DefaultDrainGrace != 285*1000000000 {
+		t.Fatalf("unexpected production drain grace: %s", supervisor.DefaultDrainGrace)
+	}
+}