ci: add permissions block to release workflow

Explicitly grant the minimum required permissions so the workflow
keeps working even after GITHUB_TOKEN's default permissions are
switched to read-only in the future.

- contents: write (required for git tag push / gh release create)

Author: shigemk2

.github/workflows/release.yml

@@ -5,6 +5,11 @@ on:
     branches:
       - main
 
+# NOTE: GITHUB_TOKEN に必要最小限の権限のみを付与
+# - contents: write → git tag push / gh release create に必要
+permissions:
+  contents: write
+
 jobs:
   release:
     runs-on: ubuntu-latest