fix: grant contents/pull-requests write permissions for update workflow

The reusable workflow's job requests contents: write and
pull-requests: write. The caller must grant at least those
permissions — permissions: {} blocks them entirely.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: devantler

.github/workflows/update-copilot-skills.yaml

@@ -5,7 +5,9 @@ on:
   schedule:
     - cron: "0 6 * * *"
 
-permissions: {}
+permissions:
+  contents: write
+  pull-requests: write
 
 jobs:
   update: