source.main moved only at a merge, so a repository never merged through the gate had no trunk head and the checker could
never judge its pull requests. The gate already hears main's head on each gitea.pull.updated (base_sha); it now adds it for
a repository source.main lacks, from the newest open pull request into main, in one compare-and-set write that never
replaces a head. A pull request into another base seeds nothing (ADR 0484 §11).