[wiki] Push publish branch explicitly (#48)

[coisa]

Summary

Fixes the publish step uncovered after PR #50. The publish job reset the local wiki branch to the PR preview commit, but EndBug/add-and-commit skipped pushing because the working tree was clean. This changes the publish step to push the selected wiki commit explicitly before deleting the preview branch.

Changes

• Replaces the publish EndBug/add-and-commit step with git push --force-with-lease origin HEAD:${WIKI_PUBLISH_BRANCH}.
• Keeps preview branch deletion after the publish push, so failed publish attempts preserve the PR wiki branch for investigation.

Testing

• git diff --check passed.
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/wiki.yml"); puts "YAML OK"' passed.
• Commit hooks passed: GrumPHP composer_script, composer, phplint, jsonlint, and yamllint.
• Verified from the previous publish log that EndBug/add-and-commit reported pushed: false when the working tree was clean.

Closes #48