fix: OpenCode plugin not discovering skills (#16)

OpenCode installs the repo via git and looks at the root for the plugin
entry point. Our skills/agents live inside understand-anything-plugin/,
so OpenCode couldn't find them.

- Add .opencode/plugins/understand-anything.js that uses the config hook
  to inject understand-anything-plugin/skills/ into skills.paths
- Add "main" field to root package.json pointing to the plugin entry
- Update INSTALL.md with version pinning and troubleshooting sections

Closes #16

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.opencode/plugins/understand-anything.js

@@ -0,0 +1,25 @@
+/**
+ * Understand Anything plugin for OpenCode.ai
+ *
+ * Auto-registers the skills directory so OpenCode discovers all
+ * understand-anything skills without manual symlinks or config edits.
+ */
+
+import path from 'path';
+import { fileURLToPath } from 'url';
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+
+export const UnderstandAnythingPlugin = async ({ client, directory }) => {
+  const skillsDir = path.resolve(__dirname, '../../understand-anything-plugin/skills');
+
+  return {
+    config: async (config) => {
+      config.skills = config.skills || {};
+      config.skills.paths = config.skills.paths || [];
+      if (!config.skills.paths.includes(skillsDir)) {
+        config.skills.paths.push(skillsDir);
+      }
+    },
+  };
+};