Merge lane: jaah

Merge lane run: batch the queued PRs, run the full gate (if any) once on their merged tree, and merge the batch when it is green. Filed by the control plane (`app/merge_lane.py::ensure_lane`).

Merged #6567, #6568, #6569, #6570, #6571; none ejected. develop was red (unregistered change_cloud_node_type workflow; Slack callback GET-mutation census). Fix-forward #6572 merged (full gate green); develop green, Jaah Deploy success.