fix: lint errors in plugin loader

- Use errors.New instead of fmt.Errorf for simple strings
- Run gofmt on new files

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

modules/plugins/loader.go

@@ -4,6 +4,7 @@
 package plugins
 
 import (
+	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -99,7 +100,7 @@ func loadPlugin(path string) error {
 	}
 
 	if plug == nil || *plug == nil {
-		return fmt.Errorf("Plugin symbol is nil")
+		return errors.New("plugin symbol is nil")
 	}
 
 	Register(*plug)