Both. Command the author to use the style, then command a reviewer to check it. Write one skill called review-prose with your rules, and another called write prose which tells the author they will be judged by review-prose, so you only write the rules once.

What I have found is that getting a model to rewrite a badly written passage is hard, because it seems to key off what it reads. It might swap some vocabulary around ok, but it doesn't fix structures very well. So getting it close to the preferred style in the first place is better.

To take this further, if you must fix existing bad prose, write a clean-prose skill which extracts the bare structure of the prose with none of the style, hands it to an author subagent who isn't poisoned with the original bad prose, then hands the output to a reviewer subagent.

Opus 5 writing is horrendous, so I have been experimenting with improving the output!