feat(agent): add provider switching and OAuth credential support

- Add getProviderInfo() and setProvider() methods to Agent class
- Expose provider methods via AsyncAgent
- Add setLlmProviderOAuthToken() for storing OAuth credentials
- Extend ProviderConfig type with OAuth fields (oauthToken, oauthRefreshToken, oauthExpiresAt)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/agent/async-agent.ts

@@ -221,4 +221,19 @@ export class AsyncAgent {
   getMessages(): AgentMessage[] {
     return this.agent.getMessages();
   }
+
+  /**
+   * Get current provider and model information.
+   */
+  getProviderInfo(): { provider: string; model: string | undefined } {
+    return this.agent.getProviderInfo();
+  }
+
+  /**
+   * Switch to a different provider and/or model.
+   * This updates the agent's model without recreating the session.
+   */
+  setProvider(providerId: string, modelId?: string): { provider: string; model: string | undefined } {
+    return this.agent.setProvider(providerId, modelId);
+  }
 }