HookEvent
branch.opened: the first push of a branch. push: any push through the door that moves or deletes a branch. merged: a Pando merge landed a branch; a buildkite hook builds main's merge commit (merge_commit) on main (main_branch). tag: a tag pushed, moved or deleted (or made by publishing a release): tag is its name, head its commit (zeros when deleted).
Type:string
one of "push", "branch.opened", "merged", "tag"