GitHub Mobile: Rebase Stacked PRs [GA]

### Value Prop
You can now rebase an entire stack of pull requests directly from GitHub Mobile, without needing to switch to a desktop browser. Track rebase progress and status in real time from the stack merge section, so you stay in control of your stacked pull request workflows wherever you are.

### Expected Outcome
Developers working with stacked pull requests often hit blockers when branches fall out of date—and resolving them previously required leaving the mobile experience entirely. This release brings full rebase support to GitHub Mobile, helping teams keep their stacks current and their momentum going, even when they're away from their desks.

该 issue 描述了 GitHub Mobile 对堆叠 pull request 进行 rebase 的支持，但没有指定源文件、测试或贡献入口。首先定位 GitHub Mobile 的实现，以及现有的 stack merge 或 rebase 工作；完成标准是该功能可在移动端体验中使用，并且能够看到 rebase 的进度和状态。

由索引模型根据 Issue 内容生成。