[workflow] Add reports preview concurrency and cleanup (#68)

[coisa]

Summary

• Add PR-scoped concurrency for reports previews so newer pushes cancel older in-progress preview runs for the same PR.
• Add scheduled/manual cleanup for stale gh-pages preview directories whose pull requests are already closed.
• Keep production reports deployments on a separate non-canceling concurrency group and update docs for the reports cleanup lifecycle.

Testing

• ruby -e 'require "yaml"; %w[.github/workflows/reports.yml resources/github-actions/reports.yml].each { |f| YAML.load_file(f); puts "#{f}: ok" }'
• git diff --check
• composer dev-tools docs

Note: actionlint is not installed in this local environment, so workflow validation used YAML parsing plus documentation generation.

Closes #68

[github-actions]

🚀 Preview is available for this pull request.

• Docs: https://php-fast-forward.github.io/dev-tools/previews/pr-88/
• Coverage: https://php-fast-forward.github.io/dev-tools/previews/pr-88/coverage/