feat: custom provider support with streamSimple

- Add resetApiProviders() to clear and re-register built-in providers
- Add createAssistantMessageEventStream() factory for extensions
- Add streamSimple support in ProviderConfig for custom API implementations
- Call resetApiProviders() on /reload to clean up extension providers
- Add custom-provider.md documentation
- Add custom-provider.ts example with full Anthropic implementation
- Update extensions.md with streamSimple config option

packages/ai/src/index.ts

@@ -8,6 +8,7 @@ export * from "./providers/google-gemini-cli.js";
 export * from "./providers/google-vertex.js";
 export * from "./providers/openai-completions.js";
 export * from "./providers/openai-responses.js";
+export * from "./providers/register-builtins.js";
 export * from "./stream.js";
 export * from "./types.js";
 export * from "./utils/event-stream.js";