Implement global automated-DM spam-mute guard

After Slava says "stop spam" (or equivalent), suppress all non-incident automated outbound DMs for 24h. Clear the mute when Slava sends any new message. Deliver: script ~/agents/slava-agent/bin/check-dm-mute.sh, marker file ~/agents/slava-agent/.dm-mute-until, and HEARTBEAT.md updates wiring the guard before any automated DM send.