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 #6535 (task 8703, head bc7f44c3 → a10e43c6) after quick gate green on merged tree (5899 passed). Nothing ejected. Queue empty, develop not red. Batch 51s (gate 34s). Memory: none applied.