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Add GitHub Actions workflows for PHP CodeSniffer and PHP Lint (8.2/8.3/8.4).

.github/workflows/php-lint.yml

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+name: PHP Lint
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        php-version: ['8.2', '8.3', '8.4']
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup PHP ${{ matrix.php-version }}
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php-version }}
+
+      - name: Lint PHP files
+        run: find . -name '*.php' -not -path './vendor/*' -exec php -l {} + | (! grep -v "No syntax errors")

.github/workflows/phpcs.yml

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+name: PHP CodeSniffer
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  phpcs:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.2'
+          tools: composer
+
+      - name: Install dependencies
+        run: composer config allow-plugins.roundcube/plugin-installer true && composer install --prefer-dist --no-progress
+
+      - name: Run PHP CodeSniffer
+        run: composer phpcs