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`).

merge lane batch 18 PRs: 15 merged, 0 failed, 1 requeued, 2 skipped, gate 4×, tests 31494 passed