Add actions:read permission to pr-playground workflow

The github-script step calls the Actions API to list workflow runs
and download artifacts, which requires actions:read permission.

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

Author: hsbt

.github/workflows/pr-playground.yml

@@ -8,6 +8,7 @@ on:
 
 permissions:
   contents: read
+  actions: read
 
 jobs:
   post-summary: