[wiki] Publish merged docs to wiki master (#48)

[coisa]

Summary

Corrects the follow-up publish failure from PR #49. The wiki repository publishes from master, not main, so the merged-PR publish job now fetches, switches, and pushes the configured wiki publish branch.

Changes

• Adds WIKI_PUBLISH_BRANCH=master to the publish job.
• Uses that branch when fetching the wiki publish ref, switching locally, and pushing the final wiki content.
• Renames the publish step labels away from main so the workflow matches the actual wiki repository branch.

Testing

• git diff --check passed.
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/wiki.yml"); puts "YAML OK"' passed.
• Verified .github/wiki remote exposes refs/heads/master and not refs/heads/main.
• Commit hooks passed: GrumPHP composer_script, composer, phplint, jsonlint, and yamllint.

Closes #48

[coisa]

Added the preview-branch cleanup step after publishing to the wiki publish branch. It runs only after the publish push succeeds, so a failed publish keeps the pr-* branch available for debugging/retry.