feat(pm): a PR over 5,000 changed lines lands only by a human merge — size predicate in check-governed-merges --test, the same reading in dispatch-gates, one rule line in SKILL.md and landing-operations

feat(pm): a PR over 5,000 changed lines lands only by a human merge — size predicate in check-governed-merges --test, the same reading in dispatch-gates, one rule line in SKILL.md and landing-operations #8400