refactor(capture): screen capture in subprocess

- Move screen capture to child process
- Isolate GDI/BitBlt crashes from agent
- Use -capture flag for internal helper mode
- Add protocol for frame request/response
- Make monitor enumeration thread-safe

lib/instance/args_test.go

@@ -7,7 +7,7 @@ import (
 
 func TestFilterArgsDropsLauncherFlags(t *testing.T) {
 	t.Parallel()
-	got := filterArgs([]string{"-ensure", "-background", "-foreground", "-addr", "0.0.0.0:5033"})
+	got := filterArgs([]string{"-ensure", "-background", "-foreground", "-capture", "-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)