docs/offscale-plugin.md becomes plugins/offscale/PLUGIN.md. most of it is about
the plugin system generally rather than about offscale, which is exactly why it
belongs with the worked example — the reasoning is most useful beside the code
it produced, and the platform's docs should not carry the history of something
it no longer knows exists.
every reference updated.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
