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 #6631, #6630, #6632 into develop (quick gate on merged tree: 5977 passed); 0 ejected. Queue empty, develop not red. Memory: none applied.