Document OpenAPI generation design

[sudomateo]

We've been making some changes to the OpenAPI generation logic per RFD-614 but we should document our design in this repository so that both humans and LLMs can benefit from it. We don't want to make a decision today and then next week forget the decision.

Things to cover.

• How OpenAPI concepts (e.g., nullable, required) translate into Go types.
• How we handle enums.
• When we decide to use pointer or value receivers for methods.

[jmcarp]

Good content for DESIGN.md. I asked Claude to summarize how we handle pointer/value and omitempty/omitzero decisions earlier, and the result made me think we need to think about it more.

[sudomateo]

Agreed. I want to give this repository a bit of a once over in terms of documentation and AI context once I finish some of my open items. DESIGN.md has a ton of good content already!