Add a /slow mode for lower-cost, slower processing

[acentrist]

What variant of Codex are you using?

Codex CLI

What feature would you like to see?

I’d like a /slow mode that is 0.5x cost for slower processing.

[etraut-openai]

I think this is already captured in #2916, which has quite a few upvotes already. If you think that it doesn't quite capture what you're looking for, please post in that thread.