Bump actions/checkout to v6

v4 ran on Node.js 20, which GitHub deprecated.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: cinderblock

.github/workflows/test.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - uses: oven-sh/setup-bun@v2
         with: