Add check_opencode_plugins() to read ~/.config/opencode/opencode.json,
parse the plugin[] array, and cross-reference against
~/.cache/opencode/node_modules/ to report installed vs. missing plugins.
Also adds opencode_config_path, opencode_cache_dir, and
has_project_opencode_config helpers.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>