Add $ARGS and Image Support for Custom Prompts

Question

### What variant of Codex are you using?

CLI

### What feature would you like to see?

Right now, any custom /prompt ignores everything after the prompt. It's a bit clunky in that regard because often you need to pass in some other information alongside the prompt. As I understand, there's a way to do it, but it seems like it would be much easier to pass it in plaintext.

Like this https://github.com/openai/codex/pull/10269

Thank you!

### Additional information

_No response_