Support the `/model` command

Users seem to enjoy the /model command in the TypeScript CLI, so we should support this in the Rust CLI, as well.

Though in the TypeScript CLI, it is a bit jarring because if you try to use this command after you have started your conversation, you hit this:

!Image

In the Rust TUI, with the advent of the /new command, we should hopefully be able to support this in a more elegant way. (Once https://github.com/openai/codex/issues/1255 is implemented, it should be even smoother.)

Also, the TypeScript CLI appears to rewrite your config.json with the new model value when you switch. If we maintain that behavior, we should probably make it clear to the user that that is happening. One small quality-of-life improvement we can provide in the Rust implementation, though, is to use https://crates.io/crates/cargo-edit to update config.toml so that we make a minimal edit to the user's config file when updating the value of model, preserving any existing comments or formatting they have.