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 #6417 (Task 8339) at c9cdf2a6 after quick gate green (5889 passed); 0 ejected; queue empty. It fixes the e2e test that made develop red at f1524694; develop's base check covers the new head. Time 56s (gate 35s). Memory: none applied.