Enhanced /review feature for pull request reviews

What feature would you like to see?

The review feature in #3774 works well when you have local changes that are accessible through a simple git status --short. I would like to have a review feature that by default would do a git diff main-branch...feature-branch where main-branch could be develop, master or main.

The use case is when you need to checkout on a PR and review it locally this feature could be useful. I have already created a custom prompt to do this inside my own codex /reviewpr:

```reviewpr.md
I want you to review the pull request associated to the current <branch-name> and provide a review of the changes. You should:

• First do a git diff develop...<branch-name> to see the changes (three dots are important)
• Then go through the code changed and highlight any issues you see

To generalize the above one would need to pass the branch you would like to compare your current branch against so something like `/reviewpr <main-branch>` would make sense to me. In terms of implementation steps I would do something close to #3774:  

- Add a new slash command in `codex-rs/tui/src/slash_command.rs` named `SlashCommand::ReviewPR`
- This command would have a template prompt with a placeholder for the <main-branch> that would be passed in the first argument of the slash command. 
- From what I know of the codebase dynamic arguments can not be passed at the moment in SlashCommand so that feature would need to be implemented

### Are you interested in implementing this feature?

I would be interested in implementing this feature :)

### Additional information

_No response_