fix: use bundle exec htmlbeautifier in bin/build

Ensures the bundled gem is used rather than whatever is in PATH,
which may not exist on a fresh CI runner.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: cb341

bin/build

@@ -2,4 +2,4 @@
 set -e
 
 bundle exec jekyll build
-find _site -name "*.html" -exec htmlbeautifier {} \;
+find _site -name "*.html" -exec bundle exec htmlbeautifier {} \;