chore: migrate CI from Bitbucket Pipelines to GitHub Actions

Replace bitbucket-pipelines.yml with a GitHub Actions workflow that runs
PHP CodeSniffer (composer lint). Follow the workflow style used in
appleple/custom-field-maker (setup action + matrix).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 1000-x-t30

.github/workflows/php.yml

@@ -0,0 +1,26 @@
+name: PHP CI
+
+on: [push]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        php-version: [8.3]
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Use PHP ${{ matrix.php-version }}
+      uses: shivammathur/setup-php@v2
+      with:
+        php-version: ${{ matrix.php-version }}
+        tools: composer
+    - name: Install dependencies
+      run: |
+        composer install --no-progress --prefer-dist
+    - name: Lint
+      run: |
+        composer lint