Support simulating/testing identity & auth logic in local dev before deploying to AWS

## Summary

Customers want to **simulate and test identity/auth logic in the local dev environment before deploying to AWS**. Today, if an agent isn't deployed to AWS, the auth process can't be exercised locally — even when existing identity resources are available.

## Problem

- Testing the auth/identity flow currently requires a full deployment to AWS.
- There's no way to validate identity logic locally against existing identity resources.
- This slows the dev loop: developers must deploy just to verify auth behaves as expected.

## Desired behavior

- Ability to run/simulate the agent's identity and auth logic in the local dev environment.
- Local testing should be able to leverage existing identity resources so the auth process can be exercised without a deployment to AWS.

## Environment

- CLI version: `0.22.0`
- User agent: `AgentCoreCLI/0.22.0 (darwin 25.5.0; node/v24.2.0)`
- OS: `darwin 25.5.0`
- Node: `v24.2.0`

No files, tests, or entry points are identified in the issue. Start by locating the CLI's local development and identity/authentication flows, then determine how existing AWS identity resources can be used without deployment. Done means developers can exercise and validate the agent's identity and auth logic locally against those resources.

Written by the indexing model from the issue text.