redesign plugin marketplace; simplify API

- Updated the plugin installation API to fetch and augment plugin discussions from GitHub, improving the index response with discussion links.
- Refactored the plugin installation UI to include better handling of plugin details, including management buttons for installed plugins.
- Improved the layout and styling of the plugin browsing interface, including search and sort functionalities, and added a summary of available plugins.

webui/components/plugins/plugin-settings.html

@@ -9,7 +9,7 @@
     <div x-data>
         <template x-if="$store.pluginSettings">
             <div x-create="$store.pluginSettings.onModalOpen()"
-                 x-destroy="$store.pluginSettings.cleanup(); window.dispatchEvent(new Event('plugin-modal-closed'))">
+                 x-destroy="$store.pluginSettings.cleanup(); $store.pluginListStore?.refresh()">
 
                 <!-- Context toolbar: Project + Agent profile (only when at least one scope is configurable) -->
                 <div class="plugin-settings-scope-section"