Fix Slack DM read in heartbeat — channel_not_found on target U066K2677F1

The heartbeat Step 3 DM read fails with channel_not_found when using target=U066K2677F1. Use Slack conversations.open with that user ID to obtain a stable DM channel ID, update HEARTBEAT.md and any cron prompts to use the channel ID directly, and verify the exact heartbeat Step 3 invocation returns messages. Do not close until the next real heartbeat successfully reads DMs.