ci: add unit test workflow

Adds GitHub Actions workflow for running unit tests across multiple PHP
versions (7.4, 8.1, 8.2, 8.3). Unit tests run without WordPress using
Brain Monkey, enabling faster feedback on pure PHP logic.

Workflow is hardened with:
- Minimal permissions (contents: read)
- Pinned action versions with SHA
- persist-credentials: false

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: GaryJones

.github/workflows/unit.yml

@@ -0,0 +1,65 @@
+name: Unit Tests
+
+on:
+  push:
+    branches:
+      - develop
+      - trunk
+    paths:
+      - '**.php'
+      - 'composer.json'
+      - 'phpunit.xml.dist'
+      - '.github/workflows/unit.yml'
+  pull_request:
+    paths:
+      - '**.php'
+      - 'composer.json'
+      - 'phpunit.xml.dist'
+      - '.github/workflows/unit.yml'
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  unit-tests:
+    name: PHP ${{ matrix.php }}
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+
+    strategy:
+      fail-fast: false
+      matrix:
+        php:
+          - '7.4'
+          - '8.1'
+          - '8.2'
+          - '8.3'
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+        with:
+          persist-credentials: false
+
+      - name: Setup PHP ${{ matrix.php }}
+        uses: shivammathur/setup-php@44454db4f0199b8b9685a5d763dc37cbf79108e1 # 2.36.0
+        with:
+          php-version: ${{ matrix.php }}
+          coverage: none
+
+      - name: Install Composer dependencies
+        uses: ramsey/composer-install@3cf229dc2919194e9e36783941438d17239e8520 # 3.1.1
+
+      - name: Setup problem matchers for PHP
+        run: echo "::add-matcher::${{ runner.tool_cache }}/php.json"
+
+      - name: Setup problem matchers for PHPUnit
+        run: echo "::add-matcher::${{ runner.tool_cache }}/phpunit.json"
+
+      - name: Run unit tests
+        run: composer test:unit