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

Start in the gh stack CLI's stack and prompt navigation entry points, tracing where arrow-key selection movement is handled. Add Ctrl+N and Ctrl+P navigation with the same direction and behavior as the arrow keys, then verify both bindings move selection correctly.

Written by the indexing model from the issue text.