Add n/p navigation to PR list buffers

[jonathanchu]

Description

Adds n/p keybindings for navigating between item rows in all list buffers (PR list, PR status dashboard, PR checks). Navigation is intuitive and will skip headers, separators, and blank lines, only landing on actual item PR rows. This follows the standard Emacs convention for read-only list buffers (e.g. Magit sections).

Closes #3

Checklist

• My elisp byte-compiles cleanly (M-x byte-compile-file)
• I've run M-x checkdoc and addressed its feedback
• I've run package-lint and addressed its feedback
• I've tested this with a recent version of Emacs

Testing

Verified locally n/p works in PR list buffer and PR status dashboard, skipping the section boundaries.

[jonathanchu]

@chopmo merging this into main.

Navigation keybinding n/p should work in all PR list buffers. Thanks again for the suggestion and let me know what you think about this implementation!

[chopmo]

@jonathanchu Thanks for adding this, works beautifully.