Update a stack before it merges

plywood switches on chant’s apply before merge.

apply:
  when: pull-request

Approve the preview, then comment:

/plywood up

The update job runs the pull request’s preview from its head in one job, under the pull request’s gate. Before it runs, it asks the forge four checks:

Check	Passes when
approved	the head has an approval
mergeable	the forge says the pull request merges as it stands
undiverged	the head contains the target branch’s tip
checks	every check passed

apply.requires lists fewer of them. Merge once the update is done; nothing updates again on merge.

The pull request holds its locks for its whole life, and lets go of them when it merges or closes.