docs: the config-repo `main` merge gate (merge = `core` + team
`operators`, approvals = `operators`), the operator merge in the forge
UI and what it deploys, the hand-added `operators` membership, and that
with no eval-verify a failed rebuild leaves `applied/main` at the merged
commit. The swarm README lists the converged gate and the merge deploy.
`merged()` also requires `pull_request.base.ref == "main"`: the hive
deploys its config repo's `main`, so a merge into another branch would
only cost a forge fetch and a refusal comment (argus, #4894).
Refs #4850
a5ea015bc6