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 #6493 (Task 8564) as 1a29d2e1e after quick gate green (5885 passed, 43s); nothing ejected; queue empty. develop_red record (5cd29f72) is stale — onboarding e2e since fixed by 04ded5da5. Memory: none applied.