Add cooperator-plugin page with interactive Claude Code demos

- New page at /cooperator-plugin with animated terminal demos showing
  multi-agent collaboration (GPU borrowing, status checks, full-stack
  coordination, incident response with 4 terminals)
- Added Plugin link to navbar
- Updated announcement banners to promote the new plugin page

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: akhatua2

src/layouts/BaseLayout.astro

@@ -33,6 +33,7 @@ const navItems = [
   { label: 'Leaderboard', href: '/leaderboard' },
   { label: 'Tasks', href: '/tasks' },
   { label: 'Trajectories', href: '/viewer' },
+  { label: 'Plugin', href: '/cooperator-plugin' },
   { label: 'Blog', href: '/blog' },
 ];