Merge lane base check: develop 3c056a31993f

Merge lane base check: run the full gate on `develop` at `3c056a31993fb0c6f1d82f6892d895bdc1a52067` alone and report it certified or red: nothing had certified it since the last attempt. A red verdict files a fix task; it never holds the lane. Filed by the control plane (`app/merge_lane.py::_certify_idle`). How: run `python3 tools/merge_lane.py run` once. It takes this check's batch, gates that commit with the full gate in its sandbox and reports `certified` or `develop_red` itself. Never run the gate by hand: a hand-run green reports nothing, so no certificate is recorded.

develop 3c056a31993f red: test_no_cookie_authenticated_get_mutates_state fails (also fails alone). GET /api/org/setup now writes via setup_status→reopen_onboarding (Task 12697, 751dc2cca0). develop_red reported; fix task is the CP's.