fix(mcp): include server name and cause in Close() errors

Previously Close() discarded all underlying errors and returned only
'failed to close N server(s)', making debugging impossible.

Now each error wraps the server name and original cause, and all errors
are joined so callers can inspect the full failure list.

pkg/mcp/manager.go

@@ -456,14 +456,14 @@ func (m *Manager) Close() error {
 					"server": name,
 					"error":  err.Error(),
 				})
-			errs = append(errs, err)
+			errs = append(errs, fmt.Errorf("server %s: %w", name, err))
 		}
 	}
 
 	m.servers = make(map[string]*ServerConnection)
 
 	if len(errs) > 0 {
-		return fmt.Errorf("failed to close %d server(s)", len(errs))
+		return fmt.Errorf("failed to close %d server(s): %w", len(errs), errors.Join(errs...))
 	}
 
 	return nil