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 16 PRs (#6251–#6268), none ejected. Fix-forward #6267 merged: lane tolerates the released jaah-forge (no --ignore-check) and clears develop's mypy regressions. Its deploy succeeded; every lane quick gate was green.