[wiki] Fix branch-protection publishing flow (#48)

[coisa]

Summary

Adopts the branch-protection-friendly wiki publishing flow for issue #48. Pull request updates now generate a PR-scoped wiki branch and commit the wiki submodule pointer back to the PR branch, while merged PRs targeting main publish the PR wiki branch into the wiki main branch.

Changes

• Split .github/workflows/wiki.yml into PR preview and merged-PR publish jobs.
• Updated .github/wiki to the generated wiki preview commit already present in this checkout.
• Updated resources/github-actions/wiki.yml so consumer stubs call the reusable workflow on PR preview and merged-PR publish events instead of only pushing from main.

Testing

• git diff --check passed.
• ruby -e 'require "yaml"; %w[.github/workflows/wiki.yml resources/github-actions/wiki.yml].each { |file| YAML.load_file(file) }; puts "YAML OK"' passed.
• Commit hooks passed: GrumPHP composer_script, composer, phplint, jsonlint, and yamllint.
• composer dev-tools could not complete because the current lock/vendor state is missing twig/twig: Composer reports that twig/twig is required by composer.json but is not present in composer.lock.

Closes #48