refactor(website): convert to ES modules with TypeScript

- Replace all inline scripts with TypeScript modules
- Create page-specific modules for agents, prompts, instructions, skills, collections, index
- Create core modules: utils.ts, search.ts, modal.ts, choices.ts, jszip.ts, theme.ts
- Remove window global exports, use proper ES module imports
- Add type interfaces for all data structures
- Use data-base-path attribute on body for base URL handling
- Bundle Choices.js and JSZip via npm instead of CDN
- Astro pages now just have single script import each

website-astro/astro.config.mjs

@@ -0,0 +1,14 @@
+import { defineConfig } from 'astro/config';
+import sitemap from '@astrojs/sitemap';
+
+// https://astro.build/config
+export default defineConfig({
+  site: 'https://github.github.io',
+  base: '/awesome-copilot/',
+  output: 'static',
+  integrations: [sitemap()],
+  build: {
+    assets: 'assets',
+  },
+  trailingSlash: 'always',
+});