refactor: convert skills-core and plugin to pure ESM for module compatibility

lib/skills-core.js

@@ -1,6 +1,6 @@
-const fs = require('fs');
-const path = require('path');
-const { execSync } = require('child_process');
+import fs from 'fs';
+import path from 'path';
+import { execSync } from 'child_process';
 
 /**
  * Extract YAML frontmatter from a skill file.
@@ -199,7 +199,7 @@ function stripFrontmatter(content) {
     return contentLines.join('\n').trim();
 }
 
-module.exports = {
+export {
     extractFrontmatter,
     findSkillsInDir,
     resolveSkillPath,