refactor: redesign plugin system and enhance message handling

- Move plugin and tool-related classes to a dedicated `Plugin-API` module.
- Introduce a new plugin lifecycle (`onInit`, `onEnable`, `onDisable`) and a more robust `Tool` API.
- Refactor `Core` to use `PrintAdvanceMessageHandler` for improved handling of assistant messages, tool responses, and errors.
- Rename `PluginLoader` to `Loader` and migrate it to `me.zacharias.chat.plugin.loader`.
- Update build configurations and Java toolchains across modules.
- Clean up obsolete plugin annotations and interfaces in the `Core` module.

.idea/misc.xml

@@ -7,7 +7,7 @@
   <component name="FrameworkDetectionExcludesConfiguration">
     <file type="web" url="file://$PROJECT_DIR$" />
   </component>
-  <component name="ProjectRootManager" version="2" languageLevel="JDK_21" project-jdk-name="26" project-jdk-type="JavaSDK">
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_26" default="true" project-jdk-name="26" project-jdk-type="JavaSDK">
     <output url="file://$PROJECT_DIR$/out" />
   </component>
 </project>