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`).

Queue had 1 PR: 0 merged, #6146 (Task 7602) ejected because it conflicts with develop 085f6da (merge_state dirty). Develop green: the vale and ast-grep-pin stages that failed at c2ad861 pass on head. Queue empty.