Add heartbeat rule to suppress repeat blocker DMs within 24h

Subtask of the 2026-08-07 reflection. Update HEARTBEAT.md Step 3a / DM logic so any heartbeat about to send a blocker DM first checks the last 24h of outbound DMs to Slava on that topic. If one was already sent, suppress the repeat regardless of .dm-mute-until. Include a test/dry-run path and update memory. This closes the remaining open hygiene item from the auth-blocker spam incident.