Convert remaining fmt.Fprintf to slog.Error in load_plugins.go

Address review feedback from TerryHowe and gjenkins8:
Convert fmt.Fprintf(os.Stderr, ...) on line 66 to slog.Error()
for consistent structured logging throughout the file.

pkg/cmd/load_plugins.go

@@ -63,7 +63,7 @@ func loadCLIPlugins(baseCmd *cobra.Command, out io.Writer) {
 	}
 	found, err := plugin.FindPlugins(dirs, descriptor)
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "failed to load plugins: %s\n", err)
+		slog.Error("failed to load plugins", "error", err)
 		return
 	}