Support j/k for navigation in gh stack view

[denniscual0217]

Summary

gh stack view stack view binds only the arrow keys for moving the cursor between branches. It would be cool that we can also bind those navigations to vim keys j|k.

I've implemented this on a branch in my fork: denniscual0217@017db08

Tested here:

Screen.Recording.2026-08-06.at.9.31.50.PM.mov

Environment

• gh version: 2.97.0
• gh stack version: 0.1.0
• macOS (arm64)