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 #6480 (Task 8557) as fa1b56595, quick gate green (5878 passed). It fixes the red develop test test_no_route_lets_a_person_leave_an_org_themselves; a focused full gate on the merged tree passed. Nothing ejected; queue empty. Memory: none applied.