Implement idempotent cleanup behavior

internal/fileops/cleanup.go

@@ -13,6 +13,9 @@ import (
 func RemoveAllUnderRoot(rootPath, target string) error {
 	root, targetName, err := openCleanupRoot(rootPath, target)
 	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return nil
+		}
 		return err
 	}
 	defer func() { _ = root.Close() }()