fix(plugin-loader): fail-fast when install_dependencies returns False

Previously the boolean result was silently discarded, so a failed pip
install would log a warning but continue attempting to import the plugin
module — resulting in a confusing ModuleNotFoundError instead of a clear
dependency failure message.

Now raises PluginError with plugin_id and plugin_dir if dependency
installation fails, stopping the load before the import is attempted.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/plugin_system/plugin_loader.py

@@ -609,7 +609,12 @@ class PluginLoader:
         """
         # Install dependencies if needed
         if install_deps:
-            self.install_dependencies(plugin_dir, plugin_id, plugins_dir=plugins_dir)
+            if not self.install_dependencies(plugin_dir, plugin_id, plugins_dir=plugins_dir):
+                raise PluginError(
+                    f"Dependency installation failed for plugin {plugin_id} in {plugin_dir}",
+                    plugin_id=plugin_id,
+                    context={'plugin_dir': str(plugin_dir)},
+                )
         
         # Load module
         entry_point = manifest.get('entry_point', 'manager.py')