I have been doing this a lot even without a skill, having Fable write a planning document, then spawning an Opus subagent with instructions to strictly follow the plan and report any deviance at the end. It also helps that then the plan is always saved in an md file so any future agent can look at it and see what happened.
Interestingly, this was tackled in this blog post[0] a month ago. They claim that plan files aren't token-efficient, because after reading the plan the workhorse model then reads all the relevant files anyways.
[0] https://news.ycombinator.com/item?id=48916512
That link just says the planning stage should vet the idea concretely so that the plan focuses on a solution that won’t immediately have to pivot.
And I think plan files should focus on general ideas and invariants, not do “implementation as prose”. That way they perform as mini-ADRs that are useful historically, especially to mine why the system is the way it is.
The trick is to delete all the relevant files after the plan is written.