ci: add Packagist auto-update workflow

[dimoschi]

Summary

• Adds a GitHub Actions workflow that notifies Packagist on pushes to main and on new tags
• Uses the Packagist API endpoint with PACKAGIST_USERNAME and PACKAGIST_API_TOKEN repository secrets

Setup required

1. Add PACKAGIST_USERNAME as a repository secret (Packagist account username)
2. Add PACKAGIST_API_TOKEN as a repository secret (from https://packagist.org/profile/ > API Token)

Note

The secrets are currently tied to a personal Packagist account. Ensure at least one other team member is added as a maintainer on the Packagist package page to avoid bus factor risk.