fix(cron): start scheduler so CRON_SCHEDULE handlers fire

[shawnburke]

Problem

A customer reported that the Go SDK's cron-interval handlers don't run. Investigation traced this to the agent server's cron wrapper, not the SDK surface.

server/cron/cron.go built a robfig/cron/v3 scheduler with cronLib.New() but never called Start():

func New() Cron {
    return &cronLibWrapper{ cron: cronLib.New() }  // never started
}

In robfig/cron v3, cron.New() starts with running: false. AddFunc → Schedule only appends entries to an internal slice while not running — jobs are dispatched solely by the goroutine that Start() launches:

func (c *Cron) Schedule(...) EntryID {
    if !c.running {
        c.entries = append(c.entries, entry)   // registered but dormant
    } else {
        c.add <- entry
    }
}

So CRON_SCHEDULE handlers were registered but never invoked. RUN_INTERVAL handlers were unaffected because they use their own time.NewTicker goroutine, independent of robfig/cron — which is why only the cron-schedule path was broken. Because both the Go and Python SDKs talk to this same Go agent server, cron-scheduled handlers were broken for both.

Why tests didn't catch it

TestApplyOption_CronSchedule uses a fakeCron whose Add runs the command immediately (go cmd()), so it never exercised the real "must be started" contract.

Fix

Call c.Start() in New(). Start() is idempotent, and adding/removing entries while running is fully supported (routed through the scheduler's channels).

Test plan

• Added server/cron/cron_test.go exercising the real wrapper with an @every 1s job.
  • Before fix: FAIL — cron job never fired; scheduler was not started (3s timeout)
  • After fix: PASS (0.20s)
• go test ./... across the agent module is green, including existing cron and handler tests.

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