Add configurable OAuth storage backend and respect --models in model selector

- Add setOAuthStorage() and resetOAuthStorage() to pi-ai for custom storage backends
- Configure coding-agent to use its own configurable OAuth path via getOAuthPath()
- Model selector (/model command) now only shows models from --models scope when set
- Rewrite OAuth documentation in pi-ai README with examples

Fixes #255

packages/ai/src/utils/oauth/index.ts

@@ -43,8 +43,11 @@ export {
 	type OAuthCredentials,
 	type OAuthProvider,
 	type OAuthStorage,
+	type OAuthStorageBackend,
 	removeOAuthCredentials,
+	resetOAuthStorage,
 	saveOAuthCredentials,
+	setOAuthStorage,
 } from "./storage.js";
 
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