Migrate #406 (ci.runner-placement) and #412 (ci.peak-anon-measurement) to the lifecycle
schema as exemplars; #390 and #411 confirmed prose-only (no standalone record). Dogfood the
append-only->lifecycle supersession: docs.append-only-guard moved to archive/, superseded by
new active docs.decision-lifecycle. Stand up docs/decisions/archive/ + migration-map.md.
Refs #521
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
