Add explicit permissions to GitHub Actions workflow for security

Co-authored-by: nhorton <[email protected]>

Author: Copilot

.github/workflows/ci.yml

@@ -9,6 +9,8 @@ on:
 jobs:
   test:
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
     strategy:
       matrix:
         ruby-version: ['3.0', '3.1', '3.2', '3.3']