Promote to prod

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PR Summary

Low Risk
README-only documentation with no runtime, deployment, or command behavior changes in this diff.

Overview
Adds a Nightly build section to the README so users can distinguish the stable bot from the bot that tracks master.

The new section explains that the nightly application receives unreleased changes first, includes a GitHub WARNING callout that it may break or lose data, and links an OAuth invite for the nightly Discord application (client_id=1507641818907672688).

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