workflow: Add top-level permissions

Set:
```
permissions:
  contents: read
```
as the default top-level permissions explicitly
to conform to recommended security practices e.g.
https://github.com/ossf/scorecard/blob/main/docs/checks.md#token-permissions

.github/workflows/commit-message-check.yaml

@@ -6,6 +6,9 @@ on:
       - reopened
       - synchronize
 
+permissions:
+  contents: read
+
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
   cancel-in-progress: true