fix: restore npm plugin loading for package installs

[sven1103-agent]

Summary

• add a CommonJS package entrypoint so OpenCode loaders that call the package root receive a callable plugin factory
• keep the existing ESM source entrypoint for import-based consumers and include the compatibility wrapper in published artifacts
• add a regression test that loads the package root via require() and verifies it initializes the plugin hooks

Testing

• node --test test/plugin-entrypoint.test.js test/cli.test.js test/hook-enforcement.test.js
• npm pack --json

Related

• closes Plugin fails to load in OpenCode: 'Plugin export is not a function' #57