ci: push-to-main tier runs only HEAD^1..HEAD; use the PUSH_BEFORE range (MQ now rebase-merges) #5952

Operator ruling 2026-10-22 14:57Z (handoff/arbiter-cop/noah-ruling-1457-mq-rebase-gaps.md): the merge queue rebase-merges, so one push to main carries N commits, and the push-to-main tier step (scripts/ci_test_tier.sh push arm, ci/sections.yml tier step) diffed only HEAD^1..HEAD (the last commit). Fix: diff github.event.before..github.event.after (PUSH_BEFORE..HEAD), guarded for an all-zero or missing before; mutation-check rows in ci_test_tier.sh --self-test. Audit list: cop inbox 2026-10-22 15:1xZ.