Bug fix
Fix self-update installing to wrong directory when slug differs from repo name
When a plugin's directory name differed from its GitHub repository name (e.g. yourls-plugin-manager vs YOURLS-PluginManager), the installer would always extract the new version into a directory named after the repo, leaving the original directory untouched and YOURLS loading both as separate plugins.
ypm_process_github_url now accepts an optional existing_slug parameter. The update, reinstall, and bulk-update handlers pass the known on-disk slug so the correct target directory is always used. Any stale directory left behind by a previous failed update (named after the GitHub repo) is cleaned up automatically.