Use GitHub secret for Pulumi GitHub token instead of encrypted config

Move GitHub token from Pulumi encrypted config to GitHub Actions secret
(PULUMI_GITHUB_TOKEN) for better secret management. The token is passed
via GITHUB_TOKEN env var which the Pulumi GitHub provider reads automatically.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/deploy.yml

@@ -51,6 +51,7 @@ jobs:
       - name: Deploy to Production
         env:
           PULUMI_PASSPHRASE: ${{ secrets.PULUMI_PROD_PASSPHRASE }}
+          GITHUB_TOKEN: ${{ secrets.PULUMI_GITHUB_TOKEN }}
         run: |
           echo "$PULUMI_PASSPHRASE" > passphrase.prod.txt
           make up