v8.0.0 — fold validate into instance promote

The validate component (mock exists-check + retag) is removed; instance
promote moves :latest and :stable together in one idempotent step.
Instances deliberately don't boot-smoke — basef's earned :stable is the
quality bar. Also: kickstart/summary clone literals bumped + release-check
guard; earned-stable.svg timing rot removed; README accuracy pass.
BREAKING: removes the validate component.