ci: add GitHub Actions workflow with lowest/highest dependency matrix

Run Pest and PHPStan on PHP 8.2–8.5 against both the lowest and the
highest dependency versions allowed by composer.json. composer.lock is
not committed, so the lowest job is what actually proves the declared
elephant-php ^0.2 || … || ^0.5 range (and neuron-ai ^3.0) still works.
A separate job checks formatting with Pint.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: mattmilesi

.github/workflows/ci.yml

@@ -0,0 +1,62 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  tests:
+    name: PHP ${{ matrix.php }} / ${{ matrix.dependencies }}
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+      matrix:
+        php: ['8.2', '8.3', '8.4', '8.5']
+        dependencies: [lowest, highest]
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          extensions: zip, dom, libxml
+          coverage: none
+
+      # composer.lock is not committed, so "highest" resolves the newest
+      # versions allowed by composer.json and "lowest" the oldest ones
+      # (e.g. elephant-php 0.2.0), checking the whole declared range.
+      - name: Install dependencies (${{ matrix.dependencies }})
+        run: |
+          if [ "${{ matrix.dependencies }}" = "lowest" ]; then
+            composer update --prefer-lowest --prefer-stable --no-interaction --no-progress
+          else
+            composer update --prefer-stable --no-interaction --no-progress
+          fi
+
+      - name: Tests
+        run: composer test
+
+      - name: PHPStan
+        run: composer stan -- --no-progress
+
+  lint:
+    name: Pint
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.4'
+          coverage: none
+
+      - run: composer update --prefer-stable --no-interaction --no-progress
+
+      - run: ./vendor/bin/pint --test