Generate memory session manager file when adding memory to a project

## Description
When a user adds memory to a project via `agentcore add memory`, the CLI should generate a memory session manager file in the project directory and provide instructions on how to integrate it into their agent.

## Category
Feature Request

## Acceptance Criteria
- [ ] `agentcore add memory` generates a memory session manager file in the project directory
- [ ] CLI outputs integration instructions after generating the file
- [ ] Instructions are framework-specific (e.g., different guidance for Strands vs LangGraph)
- [ ] Generated file follows the Agent Factory pattern (see #233)