Resolve correct Slack DM target for Slava

Slack DM delivery to Slava is failing: configured target D0AUBPV0FM0 resolves to channel_not_found. The known-good active thread uses user ID U066K2677F1. Use Slack conversations.open or users.lookupByEmail to obtain the correct DM channel ID, update HEARTBEAT.md and any cron prompts to use it, and verify delivery by reading channel history. Do not rely on placeholder values.