feat: OpenClaw cron adapter + recipe for mode:none jobs

[samuelagents]

Motivation

effect-once was extracted from the userland mitigation for at-least-once cron side effects — see openclaw/openclaw#84976 (impact:message-loss, duplicated cron sends). A first-class recipe (and optional thin helper) for OpenClaw mode:none cron jobs would make safe adoption a one-liner for that ecosystem.

Proposal

• Documented recipe deriving a stable key from task + date (or chat.id + message_id for inbound dedup) and wrapping the notification send.
• Optional example package showing a mode:none job that can be double-triggered safely.

Acceptance

• Runnable recipe in docs.
• Key-derivation guidance: key the logical unit of work, not the run id.
• Cross-link to [Bug]: Scheduled non-codex cron runs get re-executed on the codex primary model, duplicating side effects openclaw/openclaw#84976.