Restrict workflow permissions to contents: read

All CI jobs and the deploy workflow only need to read repo contents.
Adding explicit top-level permissions satisfies zizmor's
excessive-permissions audit.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -18,6 +18,9 @@ on:
       - ".gitignore"
       - ".github/workflows/deploy.yml"
 
+permissions:
+  contents: read
+
 jobs:
   backend-lint:
     runs-on: ubuntu-latest