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 #6573 (task 8810, sha 6e8fe992). Ejected #6574: ruff lint failed on its merged tree twice while the base passed. develop 6e8fe992 passes the quick gate (the 5299841a develop_red entry is stale). Queue empty. Memory: none applied.