refactor(ai): Simplify API with new streaming interface and model management

- Replace createLLM with getModel/getModels/getProviders functions
- Rename PROVIDERS to MODELS (internal only, not exposed)
- Add streamSimple/completeSimple for unified reasoning interface
- Update README with new API examples and comprehensive documentation
- Remove model registration (models are now fixed from build time)
- Add proper TypeScript typing for provider-specific options
- Document context serialization, cross-provider handoffs, and browser usage

packages/ai/scripts/generate-models.ts

@@ -338,7 +338,7 @@ async function generateModels() {
 
 import type { Model } from "./types.js";
 
-export const PROVIDERS = {
+export const MODELS = {
 `;
 
 	// Generate provider sections