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 #6148, #6144, #6149, #6147 (quick gate green on each merged tree, 5853 tests); 0 ejected. Earlier develop-red result (vale, ast-grep at c2ad861) was out of date: both pass on current develop. Queue empty; develop green.