fix(ci): grant contents:read so the grader can run

grade-assignment.yml was missing 'contents: read'. The reusable
auto-grade.yml requests it (actions/checkout needs it), and a reusable
workflow cannot request a permission the caller did not grant, so every
run would be startup_failure and no grader comment would post. Same fix
already applied to Week 11. Preventive here: no PRs open yet.

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