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 #6669 (Task 9117) as a96d44f1 after quick gate green on merged tree (6014 passed, 35s). 0 ejected. Queue empty; develop not red, no breaches. Batch 56s (gate 35s, merge 11s). Memory: none applied.