ci: heavy X64 jobs run only in merge_group/push; PR runs stay light (Noah 15:09Z) #5955

Noah ruling 15:09Z (17:09 Madrid): heavy jobs run only in the merge queue; PR runs stay light. Gate-class, quorum, undo = 1 revert.

Full verbatim text: /mnt/nvme-raid0/cop-inbox/handoff/arbiter-cop/noah-ruling-1509-heavy-in-mq-only.md (sha256 prefix 74ad76a36992).

Scope: .github/workflows/ci.yml (+ scripts/ci_mg_reuse.sh self-test). On pull_request (ready): a new light job pr-light (cheap checks, fat_driver self-test, FAT_SECRET guards) plus mg-reuse-self-test and PR Gate / authorize; x86-main, x86-main-advisories and workspace-test-shard do not run. ci / gate, gate, workspace-test still report and pass on a PR when pr-light passes. merge_group/push keep today's graph; an unknown or empty event counts as merge_group. Falsifier: EMG-8, merge-queue first-pass green >= 90% over the first 10 epic runs.