CI: add explicit permissions block (least privilege)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: desty2k

.github/workflows/build.yml

@@ -6,6 +6,9 @@ on:
   pull_request:
     branches: [master]
 
+permissions:
+  contents: read
+
 jobs:
   test:
     name: Test — Python ${{ matrix.python-version }} / ${{ matrix.os }}