#23707: docs(cron): proposal for cron reliability plane

## Summary This PR adds a design proposal doc for a **Cron Reliability Plane** in OpenClaw core: - deterministic scheduling semantics - bounded-latency control/read path - idempotent run-key execution contract - run status vs delivery status split - session survivability policies - phased implementation + acceptance tests Proposal file: - `docs/experiments/proposals/cron-reliability-plane.md` ## Motivation This direction is based on production operation of a 5-agent OpenClaw swarm with cron-heavy autonomous workflows, where recurring reliability pain classes match active community issues. Production environment reference: - https://labs.anvil-ai.io Related issue: - Closes #23704 Referenced issue clusters in the proposal: - #16199, #16094, #23302, #22895, #23063, #23376 ## Notes - This PR is documentation-only (design proposal), no runtime behavior changes. - Goal is to align on architecture before implementation phases.