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 21 PRs, including #6133, which makes the lane retry transient daemon 'busy' errors that had stalled it. Ejected 8 on real merge conflicts. Queue empty. Develop quick gate green on every merged tree; the develop_red flag pointed at stale c2ad861.