云同步机制建议（主要设计skills）

### Self Checks / 自检

- [x] I have read the [FAQ](https://github.com/farion1231/cc-switch#faq) section in README.
我已阅读 README 中的[常见问题](https://github.com/farion1231/cc-switch#常见问题)。

- [x] I have searched for [existing issues](https://github.com/farion1231/cc-switch/issues), including closed ones.
我已搜索过[已有的 Issue](https://github.com/farion1231/cc-switch/issues)，包括已关闭的。

### Related App / 涉及应用

General / 通用

### Problem or Motivation / 问题或动机

我发现云同步skills每次都是全量同步。同步慢并且损耗流量多。
希望单独设计skills的同步机制。
比如说，每次同步skills都是增量同步。全部增量同步过的skills全部显示在云端。用户可自行选择云同步中部分skills下载到本地。用户也可在软件自行删除云端中指定skills。

### Proposed Solution / 建议方案

_No response_

### Additional Context / 补充信息

_No response_

### Contribution / 参与贡献

- [ ] I am interested in contributing to this feature.
我有兴趣参与开发此功能。

Start by locating the existing cloud-sync and skills-management entry points, then review how synchronization currently handles skills. Define the incremental-sync format, cloud-side listing and deletion behavior, and local selection flow before implementation. Done means skills can sync incrementally, be listed in the cloud, selectively downloaded, and selectively deleted.

Written by the indexing model from the issue text.