Pin GitHub Actions to SHA for security

Update official GitHub Actions (actions/*, github/*) to use pinned
commit SHAs instead of version tags. This satisfies zizmor's
unpinned-action-reference security check.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: oschwald

.github/workflows/lint.yml

@@ -20,7 +20,7 @@ jobs:
           tools: composer
 
       - name: Checkout
-        uses: actions/checkout@v6
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
         with:
           persist-credentials: false
 

.github/workflows/test.yml

@@ -27,7 +27,7 @@ jobs:
         run: echo "::add-matcher::${{ runner.tool_cache }}/phpunit.json"
 
       - name: Checkout
-        uses: actions/checkout@v6
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
         with:
           persist-credentials: false
 

.github/workflows/zizmor.yml

@@ -15,7 +15,7 @@ jobs:
       security-events: write
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v6
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
         with:
           persist-credentials: false