Update design document to reflect current implementation

- Remove outdated implementation details (400+ lines of code examples)
- Remove Terraform configuration (now handled by CE infra repo)
- Add comprehensive section on prompt testing framework
- Update implementation status (AWS deployment is complete)
- Reorganize into high-level design document
- Add sections on design decisions and learnings
- Update next steps to reflect current priorities

The document now serves as a concise architectural reference
rather than an implementation guide.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: mattgodbolt