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.

Start by identifying the GitHub Mobile iOS and Android stack mergebox entry points; the issue does not name repository files or tests. Done means users can enable and manage auto-merge for stacked pull requests from the mobile stack mergebox on both platforms.

Written by the indexing model from the issue text.