Support Ctrl+N / Ctrl+P for list navigation

### Summary

When navigating stacks and prompts, only the arrow keys work for moving the selection up and down. Ctrl+N / Ctrl+P do not work as alternatives.

### Expected behavior

Support Unix / editor-style navigation so that:

- `Ctrl+N` moves to the next item (down)
- `Ctrl+P` moves to the previous item (up)

This matches common behavior in Vim, Emacs, VS Code, and many terminal UIs, and lets users keep their hands on the home row while working with `gh stack`.

> Optionally, supporting `Ctrl+J` / `Ctrl+K` for down/up (or allowing these to be configured) would make the experience even more familiar for Vim users.

### Actual behavior

Arrow keys move the selection, but `Ctrl+N` / `Ctrl+P` have no effect.

### Environment

- `gh` version: 2.90.0
- `gh stack` version: 0.0.2

從 gh stack CLI 的 stack 和 prompt 導覽入口開始，追蹤箭頭鍵選取移動的處理位置。新增與箭頭鍵具有相同方向和行為的 Ctrl+N 和 Ctrl+P 導覽，然後驗證這兩個繫結都能正確移動選取項目。

由索引模型根據 Issue 內容生成。