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 #6364, #6365, #6367 (quick gate green on the merged tree, 5878 tests). #6366 not merged: conflicts with develop, needs a rebase by its runner. Queue empty; develop at 0e7800cdb, base check open (task 8232). Memory: none applied.