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 #6132 (task 7598, quick gate green, 5848 tests, sha 9867a9d38). Ejected #6127 (task 7597): conflict with develop, reported to its runner. Develop green: stale develop_red stages (vale, ast-grep pin) re-run on 9867a9d38 and pass. Queue empty.