fix(ci): grant contents: read so the grade workflow loads

The reusable workflow at HackYourFuture/github-actions/.github/workflows/auto-grade.yml@main
contains a nested job that requests 'contents: read' (to checkout the
PR code). The caller job here only granted issues + pull-requests,
which caps every other permission at 'none'. Result: every PR fails
to load the grade workflow with

  The nested job 'test' is requesting 'contents: read', but is only
  allowed 'contents: none'.

Adding 'contents: read' to the caller's permissions block unblocks the
nested checkout step. No other behaviour changes.

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

Author: Lasse Benninga

.github/workflows/grade-assignment.yml

@@ -8,6 +8,7 @@ on:
 jobs:
   grade:
     permissions:
+      contents: read
       issues: write
       pull-requests: write
     uses: HackYourFuture/github-actions/.github/workflows/auto-grade.yml@main