fix(startup): make watchdog task keep agent alive

- Use Task Scheduler XML instead of /TR
- Run -ensure agent in-process, not child
- Add console close and panic protection
- Guard nil event channels after stop
- Add non-Windows stubs for startup/mutex

lib/instance/args_test.go

@@ -0,0 +1,15 @@
+package instance
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestFilterArgsDropsLauncherFlags(t *testing.T) {
+	t.Parallel()
+	got := filterArgs([]string{"-ensure", "-background", "-foreground", "-addr", "0.0.0.0:5033"})
+	want := []string{"-addr", "0.0.0.0:5033"}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("filterArgs() = %#v, want %#v", got, want)
+	}
+}