fix: package browser runtime mu-plugins safely

[chubes4]

Summary

• Routes remote browser runtime mu-plugin packages through the same server-side packaging and safe delivery path used by normal runtime plugins.
• Keeps mu-plugin handling separate only at installation time, where the package is extracted into wp-content/mu-plugins and a loader file is written.
• Updates smoke coverage to ensure generated Playground PHP does not fetch the original GitHub release URL directly.

Testing

• php -l packages/wordpress-plugin/src/class-wp-codebox-abilities.php
• php tests/smoke-wordpress-plugin.php

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Diagnosed the runtime dependency path split, drafted the Codebox fix and smoke-test updates, and ran targeted verification. Chris reviewed the issue and directed the upstream API-path fix.