Use the Rivet Actors cron API for native agentOS scheduling

Summary

Use the new Rivet Actors cron API in the native agentOS() actor instead of relying on AgentOS's in-process timer driver.

Upstream API: https://github.com/rivet-dev/rivet/pull/5427

For development and integration testing, we will test against the implementation in that PR before it is included in a Rivet release.

Background

AgentOS currently schedules jobs through TimerScheduleDriver. In the native Rivet actor, those timers and the in-memory job registry do not provide actor-native persistence across sleep or restart.

Rivet PR #5427 adds persistent recurring cron jobs, one-time schedule inspection, cancellation, and bounded execution history to Rivet Actors.

Scope

• Back native actor cron-expression schedules with c.cron.set(...).
• Back AgentOS ISO 8601 one-shot schedules with c.schedule.at(...).
• Route scheduled invocations through a built-in actor action that executes the serialized AgentOS action.
• Preserve support for native actor exec and session cron actions.
• Continue rejecting callback actions at the actor boundary.
• Keep a stable mapping between AgentOS job IDs and Rivet cron names or scheduled-event IDs.
• Back listCronJobs() and cancelCronJob() with the actor scheduler and map its metadata to the existing AgentOS API.
• Preserve or explicitly define AgentOS overlap semantics (allow, skip, and queue) with the actor action execution model.
• Ensure jobs do not keep the actor awake and survive actor sleep or restart.
• Keep the standalone/core TimerScheduleDriver behavior unchanged.
• Define the native actor's bounded-history policy and surface scheduler failures through existing cron events or logging where appropriate.

Upstream API

PR #5427 provides:

• c.cron.set({ name, expression, action, args?, timezone?, maxHistory? })
• c.cron.every({ name, intervalMs, action, args?, maxHistory? })
• c.cron.get(name), c.cron.list(), and c.cron.delete(name)
• c.cron.history(name, { limit? })
• Inspectable one-time schedules through c.schedule

AgentOS does not currently expose fixed-interval schedules, so c.cron.every(...) does not need a public mapping unless that surface is added separately.

Acceptance criteria

• Cron-expression and one-shot exec and session jobs execute through the native Rivet actor.
• Jobs can be listed and cancelled after the actor has slept or restarted.
• Job IDs, next/last-run metadata, cancellation, overlap behavior, and execution errors have defined mappings to the existing AgentOS cron API.
• Actor disposal or sleep does not unintentionally delete persistent jobs.
• Integration tests cover scheduling, execution, listing, cancellation, actor sleep/restart, invalid schedules, action failures, and bounded or disabled history.
• Integration testing uses https://github.com/rivet-dev/rivet/pull/5427 until the API is released.

Start by reading the native agentOS() actor and the existing TimerScheduleDriver, then review Rivet PR #5427 and its cron and schedule APIs. Use the integration-test requirements as the validation plan: exec and session jobs must survive sleep or restart, support listing and cancellation, define overlap and history behavior, and cover invalid schedules and failures.