Custom strategies in AgentCore memory not yet supported

## Description

AgentCore memory does not currently support custom strategies. Users should be able to define and use their own memory strategies beyond the built-in options.

## Steps to Reproduce

1. Attempt to configure a custom memory strategy in AgentCore
2. Observe that there is no mechanism to define or register custom strategies

## Expected Behavior

Users should be able to define custom memory strategies and use them with `agentcore add memory`.

## Actual Behavior

Only built-in strategies are supported. Custom strategies cannot be defined or used.

## CLI Version

_N/A_

## Operating System

macOS