the design doc moves into the plugin it produced

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>

plugins/offscale/manifest.ts

@@ -27,7 +27,7 @@ export const manifest: PluginManifest = {
   // `[open]` What a member's grant MEANS here is this plugin's own job and is not finished. The queries
   // still scope by the caller (`listHeadscaleServers(userId)`), so a granted member would see their own
   // empty server list rather than the owner's, and could register a Headscale of their own. The model in
-  // docs/offscale-plugin.md is one shared resource: read sees what the owner sees, write can change it.
+  // ./PLUGIN.md is one shared resource: read sees what the owner sees, write can change it.
   // That is a change inside these queries, not a flag on the manifest.
   //
   // Worth knowing while it is unfinished: the stored credential is a Headscale ADMIN api key that can