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

[jesseturner21]

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