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 #5913, #5799, #5924, #5905, #5896, #5925 (#5905/#5896 after runners rebased past develop conflicts). #5923 closed by its owner. Nothing ejected; queue empty; develop CI green.