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 PR #6710 (task 9224) as 8a8156d after its merged tree passed the quick gate (6003 tests passed, 0 failed). Nothing ejected. Queue is empty and develop is not red. Batch took 49s, 30s of it the gate. Memory: none applied.