Clear non-empty prompts with ctrl + c

[gpeal]

This updates the ctrl + c behavior to clear the current prompt if there is text and you press ctrl + c.

I also updated the ctrl + c hint text to show ^c to interrupt instead of ^c to quit if there is an active conversation.

Two things I don't love:

1. You can currently interrupt a conversation with escape or ctrl + c (not related to this PR and maybe fine)
2. The bottom row hint text always says ^c to quit but this PR doesn't really make that worse.

https://github.com/user-attachments/assets/6eddadec-0d84-4fa7-abcb-d6f5a04e5748

Fixes https://github.com/openai/codex/issues/3126