The only path to marking a runtime offline was the daemon's deregister
call on graceful shutdown. If the daemon crashed, was killed, or lost
network, the status stayed "online" forever. Add a background goroutine
that sweeps every 30s and marks runtimes offline after 45s without a
heartbeat (3 missed intervals).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>