Add Ctrl + N / Ctrl + P navigation support in Codex suggestion popups

What feature would you like to see?

When typing commands such as / (slash commands) or using @ to pick files or users, the suggestion list currently doesn’t support Ctrl + N and Ctrl + P for navigation. These keybindings are standard in terminal and editor environments (e.g., bash, vim, zsh, VS Code) for moving through autocomplete lists.

Expected behavior:

Ctrl + N → move to the next suggestion

Ctrl + P → move to the previous suggestion

Maybe also add Ctrl + Y to accept the selected suggestion, similar to pressing Tab.