chore: wire storage-check into upstream plugin infra after rebase

- Add databricks-serverless-storage-check to metaplugin/plugin.meta.json skills map
- Extend iter_skill_dirs in scripts/skillsgen/discovery.py to yield one level
  of nested sub-skill dirs (scoped to STABLE_REPO_DIR)
- Regenerate manifest.json and all plugins/databricks/ bundles

Co-authored-by: Isaac

Author: GABRIELLE DOMPREH

metaplugin/plugin.meta.json

@@ -92,6 +92,7 @@
     "databricks-pipelines": { "keyword": "pipelines" },
     "databricks-dabs": { "keyword": "dabs" },
     "databricks-serverless-migration": { "keyword": "serverless" },
+    "databricks-serverless-storage-check": { "keyword": "serverless-storage-check" },
     "databricks-vector-search": { "keyword": "vector-search" }
   },