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 #6395 (task 8292) at 5927384e after a quick gate on the merged tree came back green (5889 passed). Nothing ejected, queue empty, develop not red; base check task 8307 is running on the merge. Memory: none applied.