feat: add opencode plugin support for automatic skill registration

[gunhol]

Summary

• Adds an OpenCode plugin entry point that auto-registers the visual-explainer skill directory via a config hook — no manual file copies needed anymore
• Updates package.json with "type": "module" and "main" so OpenCode can resolve the plugin from git
• Updates README: plugin one-liner is now the primary OpenCode install method, manual copy preserved as fallback

Changes

File	Change
.opencode/plugins/visual-explainer.js	New — config hook pushes plugins/visual-explainer into config.skills.paths
package.json	Added "type": "module", "main": ".opencode/plugins/visual-explainer.js", included .opencode/ in files
README.md	Updated OpenCode section with plugin install; updated install table row

Usage

Add this to your opencode.json:

{ "plugin": ["visual-explainer@git+https://github.com/nicobailon/visual-explainer.git"] }

Restart OpenCode and the skill is auto-registered.

Follows the same pattern as obra/superpowers.