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 #6690 (task 9177) at 8b447cd after quick gate green on merged tree (6014 passed); 0 ejected. Queue empty, develop not red, no health breaches. Time 52s (gate 34s). Memory: none applied.