fix: clean up stale root-level files on gh-pages deployment

Removes old files from when book was deployed at root:
- contents/, assets/, site_libs/, tools/
- _redirects, netlify.toml, search.json, sitemap.xml, etc.

This fixes the /kits redirect issue caused by stale _redirects file.

Author: profvjreddi

.github/workflows/book-publish-live.yml

@@ -1031,6 +1031,12 @@ jobs:
           echo "🗑️ Clearing existing book/ directory..."
           rm -rf book/
 
+          # Clean up stale root-level files from old deployment structure
+          echo "🧹 Removing stale root-level files..."
+          rm -rf contents/ assets/ site_libs/ tools/
+          rm -f _redirects netlify.toml search.json sitemap.xml site.webmanifest 404.html
+          echo "✅ Stale files removed"
+
           # Create book directory and copy new content
           echo "📦 Deploying textbook to /book/..."
           mkdir -p book