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 #6605 (8921) and #6607 (8934); none ejected. develop's red billing matrix tests (culprit b07806837, Task 8913) were fixed by #6607 and are green on f30e9a3. Queue empty. Memory: none applied.