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 #6666 (Task 9039) at 770870ab8 after the quick gate passed on the merged tree (6014 tests). Nothing ejected. develop is green: the mypy red flag on 86fb5954 was stale, and mypy passes on the new head. Batch took 50s (35s of it gating). Memory: none applied.