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 #6333 (task 8181) and #6334 (task 8120) in one batch; quick gate green on the merged tree, 5888 tests passed. Nothing ejected; queue empty. develop green: the recorded mypy failure was from older sha 55e8473, and mypy passes on 7a1b57949.