fix: align plugin with current OpenCode loader

[sven1103-agent]

Summary

• switch the package to the current OpenCode v1 plugin module contract with a default export object exposing id and server()
• flatten hook registration and update handlers to mutate hook output objects in place, matching the current @opencode-ai/plugin API
• publish the plugin through ./server and oc-plugin metadata, and cover the package root/server entrypoint with regression tests

Testing

• node --test test/plugin-entrypoint.test.js test/hook-enforcement.test.js test/advisory-cleanup.test.js test/cli.test.js test/schema-contract.test.js
• npm pack --json

Related

• fixes Plugin cannot load in current OpenCode due to loader API mismatch #59