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/capture/helper_stub.go

@@ -0,0 +1,5 @@
+//go:build !windows
+
+package capture
+
+func RunHelper() int { return 1 }