fix(ci): add permissions for leaderboard PR comment posting

The validate job's github-script step was failing with 403
"Resource not accessible by integration" because the workflow
lacked explicit write permissions for pull-requests and issues.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jeremyeder

.github/workflows/leaderboard.yml

@@ -15,6 +15,11 @@ on:
   # Manual trigger for testing
   workflow_dispatch:
 
+permissions:
+  contents: write
+  pull-requests: write
+  issues: write
+
 jobs:
   # Job 1: Validate leaderboard submissions (PR only)
   validate: