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`).

0 merged. Ejected #6243 (task 7934): the tenancy bypass census failed on the merged tree, with an unlisted session-get site at routes/tasks/dispatch.py::_tell_push_as_member (apps/api/test_tenancy_bypass_sites.py:799). develop fb0da2d gated green; queue empty.