fix(docs): use brand logo as favicon, drop placeholder ico

Author: espetro

docs/.vitepress/config.ts

@@ -36,7 +36,6 @@ export default withMermaid(defineConfig({
   head: [
     ['meta', { name: 'theme-color', content: '#3c82f6' }],
     ['link', { rel: 'icon', type: 'image/png', href: '/img/logo-dark.png' }],
-    ['link', { rel: 'alternate icon', href: '/favicon.ico' }],
     ['meta', { name: 'keywords', content: 'ai agent plugin, claude code plugin, codex plugin, opencode plugin, universal agent manifest, ai harness plugin manager' }],
     ['meta', { property: 'og:type', content: 'website' }],
     ['meta', { property: 'og:url', content: DOCS_SITE }],