[workflow] Clean up wiki preview branches (#83)

[coisa]

Summary

• Add scheduled/manual cleanup for wiki preview branches whose pull requests are already closed.
• Delete the PR wiki preview branch when a pull request is closed without merge, and make post-merge preview deletion idempotent.
• Add the schedule to the packaged consumer workflow stub and document the wiki cleanup lifecycle.

Testing

• ruby -e 'require "yaml"; %w[.github/workflows/wiki.yml resources/github-actions/wiki.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 #83

[github-actions]

🚀 Preview is available for this pull request.

• Docs: https://php-fast-forward.github.io/dev-tools/previews/pr-86/
• Coverage: https://php-fast-forward.github.io/dev-tools/previews/pr-86/coverage/