Allow `/prompt` usage within custom reviews

What feature would you like to see?

I often use /review → custom instructions because although the default review uncommitted changes does an amazing job, it _can_ overlook bad practices / code smells.

With custom instructions I'm able to add a few additional pointers about what is and what isn't acceptable and what to look out for. It gets tedious to repeatedly have to type those out or copy paste etc.

If I could just use a saved /prompt like I can normally, that would save me from additional typing. With a saved prompt I could have all the 'check for this check for that' that's common between projects, and only add the bare minimum additional instructions as needed.

Additional information

_No response_