Fix heartbeat false GitHub auth blocker and DM spam

Heartbeat sessions were reading gh auth status without eval $(~/bin/gh-fleet-token.sh), concluding auth was broken, and sending repeated DMs to Slava (~10 in 18h) despite the wrapper working. Update HEARTBEAT.md to source the wrapper before every gh command, document the auth-blocker proof requirement, and set a DM mute to stop the spam.