fix: improve promote-release workflow robustness and error handling

[boscorat]

Address git-expert review feedback with the following improvements:

• Fix draft release check logic: now explicitly rejects already-live releases instead of warning and proceeding (prevents accidental re-promotion)
• Add artifact validation: verify wheel and sdist exist before publishing to PyPI
• Implement PyPI verification with exponential backoff retry (up to 5 attempts) at 10s, 25s, 40s, 55s, 70s intervals to handle slower indexing
• Fix string interpolation: use heredoc to avoid shell-to-Python interpolation issues with special characters in package names
• Reorder deployment creation: create deployment AFTER PyPI verification succeeds to ensure deployment records accurately reflect release state
• Add conditional guards: deployment status only updates if deployment created
• Simplify retroactive docs update: improve error handling, clearer git state management, explicit cleanup of stashed changes
• Add better error messages: distinguish between missing releases, non-draft releases, artifact download failures, and verification timeouts

All changes maintain backward compatibility while improving reliability and maintainability. Workflow will now handle edge cases and transient failures more gracefully.