Create Design System Expert Custom Agent

As discussed in #3040 (closed), Duo doesn't know about our design system.

The current, best way to do that is with a Custom Agent.

Let's implement one so that users can build with Duo and our design system.

The custom agent's prompt will lean heavily on links to https://design.gitlab.com, so &33 is a soft blocker of this.

Tasks

• Prepare test prompts so we can compare before/after.