feat: support GitHub App cross-repo token

[Pigbibi]

Summary

• let monthly optimization planner mint a GitHub App token for cross-repo access when app credentials are configured
• keep CROSS_REPO_GITHUB_TOKEN as a fallback so the current setup keeps working during migration
• cover the new token selection path in the planner workflow config test

Verification

• python3 -m unittest tests/test_monthly_optimization_planner_workflow_config.py
• uvx ruff check tests
• ruby YAML parse for .github/workflows/monthly_optimization_planner.yml