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 #6345 (42967d3, quick gate green, 5888 tests). Ejected #6344: the batch failed the Question-tool roster guard twice (the doctrine fixture lacks the new fleet/memory-on.md); #6345 alone was green. Develop green; the mypy red at 55e8473 was stale.