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/client_stub.go

@@ -0,0 +1,15 @@
+//go:build !windows
+
+package capture
+
+import (
+	"errors"
+
+	"tea.chunkbyte.com/kato/go-worm/lib/models"
+)
+
+func CaptureMonitor(index int, format string, quality int) (models.CapturedImage, error) {
+	return models.CapturedImage{}, errors.New("capture helper is windows-only")
+}
+
+func Stop() {}