Design API-first modeling approach

[angeloashmore]

Research and design how CLI-based modeling should work when integrated directly with the type builder API rather than local JSON files.

Key questions:

• Should field/type mutations go directly to the API (like the cloud type builder)?
• Are local model.json files still needed, or only for codegen/type generation?
• How does this interact with pull/push/sync?
• What's the right UX for adding fields, connecting slices, etc. from the CLI?