feat: use COPILOT_PAT for token probe if available

Falls back to GITHUB_TOKEN if COPILOT_PAT not set.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/llm-hello-world.yaml

@@ -156,7 +156,7 @@ jobs:
     steps:
       - name: Probe token limits
         env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GITHUB_TOKEN: ${{ secrets.COPILOT_PAT || secrets.GITHUB_TOKEN }}
         run: |
           echo "=== Token Limit Probe ==="
           echo "Testing progressively larger payloads to find the ceiling"