Add PHPUnit test suite with Brain\Monkey (v0.1.6)

Add test infrastructure for isolated unit testing without WordPress/WooCommerce:
- 27 tests (54 assertions) covering TemplateOverride and WooCommerceExtension
- Brain\Monkey for WordPress function mocking, class stubs for TwigService and WC_Product
- PHPUnit test job added to Gitea CI pipeline between lint and build-release
- Test artifacts excluded from release packages

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

.gitea/workflows/release.yml

@@ -24,10 +24,31 @@ jobs:
         run: |
           find . -name "*.php" -not -path "./vendor/*" -not -path "./node_modules/*" -print0 | xargs -0 -n1 php -l
 
+  test:
+    name: PHPUnit Tests
+    runs-on: ubuntu-latest
+    needs: [lint]
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.3'
+          extensions: mbstring, xml, dom
+          tools: composer:v2
+
+      - name: Install Composer dependencies
+        run: composer install --no-interaction
+
+      - name: Run PHPUnit
+        run: composer exec -- phpunit
+
   build-release:
     name: Build Release
     runs-on: ubuntu-latest
-    needs: [lint]
+    needs: [test]
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
@@ -88,8 +109,8 @@ jobs:
           cp -a . "${STAGING_DIR}/${THEME_NAME}"
 
           cd "${STAGING_DIR}/${THEME_NAME}"
-          rm -rf .git .gitea .github .vscode .claude releases node_modules
-          rm -f CLAUDE.md PLAN.md composer.json composer.lock .gitignore .editorconfig
+          rm -rf .git .gitea .github .vscode .claude releases node_modules tests .phpunit.cache
+          rm -f CLAUDE.md PLAN.md composer.json composer.lock .gitignore .editorconfig phpunit.xml.dist
           find . -name '*.log' -o -name '*.po~' -o -name '*.bak' -o -name '.DS_Store' | xargs rm -f 2>/dev/null || true
 
           cd "${STAGING_DIR}"