feat: 添加 s3-storage-operations 全局技能

将 S3 对象存储操作技能从本地提升为全局内置技能，
通过 builtin-skills.json 注册，随客户端同步分发。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

builtin-skills.json

@@ -4,6 +4,7 @@
     "delete-agent",
     "discover-agent",
     "manage-skills",
+    "s3-storage-operations",
     "update-agent"
   ]
 }