Or, you could create a separate issue for designing the project and for implementing it. Then cc the PM on both. Done!
At the end of the day, someone should be responsible for completing the issue. Is it a PM? Assign it to them. Is it a designer? Assign it to them.
We use Codetree as well, and it's wonderful. But the key is to use small, focused issues with a single individual accountable for each one.