feat: scope npm package as @dietrichgebert/ponytail

Publish under a scope you own outright instead of opencode-ponytail (which
sits on a third-party npm account). Matches the GitHub handle and repo.
publishConfig.access is already public, which scoped public packages need.

.opencode/plugins/ponytail.mjs

@@ -7,7 +7,7 @@
 // source of truth.
 //
 // OpenCode loads this as a server plugin — add it to your opencode.json:
-//   { "plugin": ["opencode-ponytail"] }
+//   { "plugin": ["@dietrichgebert/ponytail"] }
 
 import { createRequire } from 'module';
 import fs from 'fs';