Operator decision at the ADR 0020 acceptance: verification is
subkey-only now instead of waiting for the stated 2026-11-01 expiry.
Links in digest mails sent before the #188 key separation stop working;
recipients use the in-app notification settings. A regression test pins
that the legacy derivation (root key + purpose prefix) can never verify
again; security.md records the removal.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AUtYMxwTCMHG9mVHnwbFg8