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 PR #6323 (task 8115) as 2e87ef0a after the quick gate passed on the merged tree (5872 passed). Nothing ejected. The develop mypy red at 55e8473 was stale: mypy passes on 2e87ef0a, so develop is green. Queue is empty.