chore(deps): update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/build-antora.yml

@@ -45,7 +45,7 @@ jobs:
 
     steps:
       - name: Checkout playbook source
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Setup Node.js
         uses: actions/setup-node@v5

.github/workflows/claude.yml

@@ -26,7 +26,7 @@ jobs:
       actions: read # Required for Claude to read CI results on PRs
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           fetch-depth: 1