Plugin-dir file watcher with adaptive polling and ConfigChange hook gate

Claude Code now watches plugin skill directories (both whole folders and individual files) for changes, polling more often when there has been recent interaction and less often when idle. Before hot-reloading a plugin that uses 'files' scope, it asks a ConfigChange hook (a hook that runs when a configuration file changes during a session) whether the reload should proceed. If the hook blocks the reload, the currently loaded version of the plugin stays in place.