Opencode v2 plugin support

Summary

The graphify opencode plugin uses the v1 plugin API. Opencode v2 has a new API and a migration guide available.

Implementation

Both v1 and v2 plugins can be installed side-by-side. The v2 plugin must be installed in the .opencode/plugins (plural) folder (only read by v2), whereas v1 stays in .opencode/plugin. Alternatively, opencode 1.18.29+ / v2 also support dual-entrypoint plugins.