Plane module

Pull request preview arenas

Get one disposable arena per pull request, forked with participants hidden and a sample of the data, removed on its own even if nobody tells it to be.

previews.list reads the live and recently reaped previews this workspace holds. previews.open forks one from a source yard, redacting participant DIDs and sampling a quarter of its data by default, under a mandatory deadline. A preview is a real yard, marked ephemeral so it is never promoted back.