Check whether a branch is ready to merge

Lists every blocker, not only the first. The merge gates an ordinary merge by the caller checks: variant, editor groups, branch checks, designated and required reviewers, review policy approvals, blocking item checks and stage gates. The review page holds that the merge itself does not enforce: draft state, a merge already queued or running, and no changes. The advisory merge conflict preview; the merge runs its own exact conflict check. A gate that cannot be judged yet (a scan still running or too large) is listed under unknown, and ready is true only when both lists are empty. This read enforces nothing; the merge re-checks its own gates.