chore: do not use sentinel errors when unneeded

- main reason being it's a burden to always define sentinel errors at global scope, wrap them with `%w` instead of using a string directly
- only use sentinel errors when it has to be checked using `errors.Is`
- replace all usage of these sentinel errors in `fmt.Errorf` with direct strings that were in the sentinel error
- exclude the sentinel error definition requirement from .golangci.yml
- update unit tests to use ContainersError instead of ErrorIs so it stays as a "not a change detector test" without requiring a sentinel error

internal/portforward/loop.go

@@ -177,11 +177,9 @@ func (l *Loop) GetPortsForwarded() (ports []uint16) {
 	return l.service.GetPortsForwarded()
 }
 
-var ErrServiceNotStarted = errors.New("port forwarding service not started")
-
 func (l *Loop) SetPortsForwarded(ports []uint16) (err error) {
 	if l.service == nil {
-		return fmt.Errorf("%w", ErrServiceNotStarted)
+		return errors.New("port forwarding service not started")
 	}
 
 	return l.service.SetPortsForwarded(l.runCtx, ports)