Add support for code review custom instructions input

What version of the Codex App are you using (From “About Codex” dialog)?

Version 260205.1301 (40.0.0)

What subscription do you have?

Pro

What issue are you seeing?

/review is missing the options "a commit" and most importantly "Custom review instructions"
<img width="793" height="148" alt="Image" src="https://github.com/user-attachments/assets/4fb191ec-c72b-435d-b48d-628134af3217" />

They exist in the TUI:
<img width="342" height="172" alt="Image" src="https://github.com/user-attachments/assets/9560e518-4952-475c-badd-b8b794d72269" />

Most often I work in a folder which has multiple sibling repos, rather than working in one repo at a time. So I need to pass custom instructions to say where to look, which diff range, and what exactly to check.

What steps can reproduce the bug?

1. Open the app
2. type "/review" and select "Code review" option
3. Observe missing options

What is the expected behavior?

1. Open the app
2. type "/review" and select "Code review" option
3. Select "Custom review instructions"
4. Type in instructions
5. Begin review agent with no errors

Additional information

_No response_