Adds support for a `/diff` command comparable to the one available in the TypeScript CLI. <img width="1103" alt="Screenshot 2025-06-26 at 12 31 33 PM" src="https://github.com/user-attachments/assets/5dc646ca-301f-41ff-92a7-595c68db64b6" /> While here, changed the `SlashCommand` enum so the declared variant order is the order the commands appear in the popup menu. This way, `/toggle-mouse-mode` is listed last, as it is the least likely to be used. Fixes https://github.com/openai/codex/issues/1253.