Support modifying existing agent config (e.g. add an authorizer to an agent created without identity)

## Summary

Customers need a way to **modify existing AgentCore values/resources after creation**. In particular, when an agent is added **without identity**, there is no way to add an authorizer to that agent afterward.

## Problem

- Agent configuration is effectively fixed at creation time for some properties.
- If an agent is created without identity, an authorizer cannot be added later.
- This forces a recreate-from-scratch workflow to change something that should be editable in place.

## Desired behavior

- Ability to modify existing agent values/resources after the agent has been created.
- Specifically, allow adding an authorizer to an agent that was originally created without identity.

## 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 named in the issue. Start by locating the CLI commands and implementation for creating and updating agents, then trace how identity and authorizer configuration are validated. Done means an existing agent created without identity can receive an authorizer without being recreated.

Written by the indexing model from the issue text.