feat: GitHub updater — WordPress plugin update notifications from GitHub Releases

[welbinator]

Summary

Adds a GitHub Releases-based update system so WordPress notifies site owners when a new LearnKit version is available — enabling one-click updates directly from the Plugins page.

How it works

1. Hooks into pre_set_site_transient_update_plugins to check the GitHub API for a newer release
2. If a newer version exists, injects the update data into the WP update transient
3. Also hooks plugins_api to populate the "View version details" modal with release notes/changelog
4. Results are cached in a 12-hour transient to avoid hammering the API; cache is busted after a successful update

Improvements over prior art

Area	Improved
Caching	12-hour transient; busted after update
Error handling	Validates HTTP status code + response shape, not just is_wp_error
Asset selection	Loops assets for application/zip; falls back to GitHub's auto-generated tag zip
Plugin info popup	plugins_api filter populates "View details" modal with changelog
Debug noise	No error_log() calls left in
User-Agent	Proper WordPress/X.X; home_url() format
GitHub API version	Sends X-GitHub-Api-Version: 2022-11-28 header
Security	download_url sanitized with esc_url_raw() before storing

Files changed

• includes/class-learnkit-github-updater.php — new updater class
• learnkit.php — require the updater