chore: drop prettier, use htmlbeautifier only

bin/build runs jekyll build + htmlbeautifier on _site/.
Both CI and deploy use bin/build — no prettier anywhere.

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

Author: cb341

README.md

@@ -22,7 +22,7 @@ Visit `localhost:4000` to view the site.
 bin/build
 ```
 
-Runs `jekyll build` and formats output with `prettier`.
+Runs `jekyll build` and formats output HTML with `htmlbeautifier`.
 
 ## Check