ax(node): replace prose comment with usage example on createTarball

AX Principle 2 — comments show HOW with real values, not WHAT the
signature already says. The previous comment restated the function
signature in prose; replaced with a concrete call showing filename/
content pairs.

Co-Authored-By: Charon <charon@lethean.io>

pkg/node/bundle.go

@@ -206,7 +206,8 @@ func isJSON(data []byte) bool {
 	return data[0] == '{' || data[0] == '['
 }
 
-// createTarball creates a tar archive from a map of filename -> content.
+// tarData, err := createTarball(map[string][]byte{"xmrig": binaryData, "config.json": cfgData})
+// if err != nil { return nil, err }
 func createTarball(files map[string][]byte) ([]byte, error) {
 	var buffer bytes.Buffer
 	tarWriter := tar.NewWriter(&buffer)