add safe prune command for migrated runtime backups

README.md

@@ -61,6 +61,13 @@ Run manually:
 ~/.mosaic/bin/mosaic-link-runtime-assets
 ```
 
+Prune migrated legacy backups from runtime folders (dry-run by default):
+
+```bash
+~/.mosaic/bin/mosaic-prune-legacy-runtime --runtime claude
+~/.mosaic/bin/mosaic-prune-legacy-runtime --runtime claude --apply
+```
+
 Opt-out during install:
 
 ```bash