Bump actions/checkout to v5 (Node 24)

Silences the GitHub Actions deprecation warning about checkout@v4
targeting Node.js 20.

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

Author: molefrog

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
   check:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
 
       - uses: oven-sh/setup-bun@v2
         with: