Add explicit permissions to corpus-integrity workflow for security

Co-authored-by: bact <128572+bact@users.noreply.github.com>

Author: Copilot

.github/workflows/corpus-integrity.yml

@@ -28,6 +28,8 @@ concurrency:
 jobs:
   corpus-integrity:
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
 
     steps:
       - name: Checkout