GitHub Mobile: Auto-merge support for Stacked PRs [GA]

### Value Prop
You can now enable and manage auto-merge for stacked pull requests directly in GitHub Mobile on iOS and Android. No more switching to a desktop browser just to configure auto-merge on a stack — everything you need is available right from the stack mergebox in the app. This keeps your workflow moving whether you're at your desk or on the go.

### Expected Outcome
Developers working with stacked pull requests gain full auto-merge control on mobile, removing a key gap that previously forced context-switching to desktop. By bringing this capability to the mobile stack mergebox, GitHub Mobile becomes a more complete tool for managing complex review workflows from anywhere.

首先确定 GitHub Mobile 在 iOS 和 Android 上的 stack mergebox 入口点；该 issue 未指定 repository 文件或测试。完成标准是用户可以在两个平台上通过移动端 stack mergebox 为堆叠的 pull requests 启用和管理 auto-merge。

由索引模型根据 Issue 内容生成。