Codifies the structure ADRs 019-022 converged on, pins an
Accepted/Superseded/Deprecated lifecycle with a no-silent-rewrite rule,
adds an adr-template.md scaffold, and plans a Status-header backfill of
ADRs 001-018. Basis for ADR-023.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
