Remove yarn, use npm exclusively

Replace all yarn references with npm across README, CLAUDE.md,
CHANGELOG, .gitignore, and CI workflow. Remove yarn.lock.

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

README.md

@@ -31,8 +31,8 @@ A modern WordPress Block Theme built from scratch with Bootstrap 5.
 git clone ssh://git@src.bundespruefstelle.ch:2022/magdev/wp-bootstrap.git
 cd wp-bootstrap
 composer install
-yarn install
-yarn build
+npm install
+npm run build
 ```
 
 ## Development
@@ -40,7 +40,7 @@ yarn build
 ### Prerequisites
 
 - Node.js 20+
-- Yarn
+- npm
 - Composer
 - PHP 8.3+
 
@@ -48,10 +48,10 @@ yarn build
 
 | Command | Description |
 | --- | --- |
-| `yarn build` | Compile SCSS, minify CSS, copy Bootstrap JS |
-| `yarn dev` | Watch SCSS files and recompile on changes |
-| `yarn scss` | Compile SCSS only |
-| `yarn postcss` | Minify CSS with Autoprefixer |
+| `npm run build` | Compile SCSS, minify CSS, copy Bootstrap JS |
+| `npm run dev` | Watch SCSS files and recompile on changes |
+| `npm run scss` | Compile SCSS only |
+| `npm run postcss` | Minify CSS with Autoprefixer |
 | `composer install` | Install PHP dependencies |
 
 ### Project Structure