Fix Stop() not stopping the cron scheduler - #86

Summary

Start built the job scheduler into a local variable and started it, but never assigned it to the node. Stop only ever calls n.crons.Stop(), so that check was always a no-op. Every cron job kept running after Stop, including health checks, sync status, node version and peer polling, and the scheduler goroutine itself leaked.

Fix assigns the scheduler to the node before starting it so Stop can actually stop it.

Test plan

• Added TestStopStopsScheduler in pkg/beacon/beacon_test.go, running a real Start/Stop cycle against a fake client and checking the scheduler is actually stopped
• Confirmed the test fails against the old code and passes against the fix
• go build ./..., go vet ./..., go test ./... all green (this test is intentionally not run under -race: it hits an unrelated, pre-existing race inside the wallclock dependency's own ticker goroutine that triggers on any full Start() call, independent of this fix, and this repo's CI does not run tests with -race either)