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.go

@@ -14,6 +14,8 @@ func filterArgs(args []string) []string {
 			continue
 		case lower == "ensure" || strings.HasPrefix(lower, "ensure="):
 			continue
+		case lower == "capture" || strings.HasPrefix(lower, "capture="):
+			continue
 		}
 		out = append(out, arg)
 	}