Clarify MemoryClient deprecation status - documentation inconsistency

## Summary

The documentation for `MemoryClient` has inconsistent messaging about its status:

1. [`src/bedrock_agentcore/memory/README.md#L39`](https://github.com/aws/bedrock-agentcore-sdk-python/blob/main/src/bedrock_agentcore/memory/README.md#L39): "MemoryClient - Legacy client interface (still supported but not recommended for new projects)"
2. [`src/bedrock_agentcore/memory/README.md#L120`](https://github.com/aws/bedrock-agentcore-sdk-python/blob/main/src/bedrock_agentcore/memory/README.md#L120): "While still supported, we recommend migrating to MemorySessionManager for new projects"
3. [`src/bedrock_agentcore/memory/README.md#L555`](https://github.com/aws/bedrock-agentcore-sdk-python/blob/main/src/bedrock_agentcore/memory/README.md#L555): "MemoryClient: Legacy client interface (deprecated)"

The first two say "still supported" while the third says "deprecated". Additionally, there's no actual `DeprecationWarning` in the code when users instantiate [`MemoryClient`](https://github.com/aws/bedrock-agentcore-sdk-python/blob/main/src/bedrock_agentcore/memory/client.py#L43).

## Proposed Options

### Option A: Formally deprecate MemoryClient
- Add `DeprecationWarning` to `MemoryClient.__init__()`
- Update all documentation to consistently say "deprecated"
- Set a target removal version (e.g., v2.0.0)

### Option B: Keep as "legacy" without formal deprecation
- Remove "deprecated" from API Reference section
- Keep "legacy" and "not recommended" language
- No code changes needed

### Option C: Staged approach
- Fix documentation to be consistent ("legacy, not recommended")
- Add deprecation warning in a future minor release
- Remove in a future major release

まず、src/bedrock_agentcore/memory/README.md にある MemoryClient への3つの参照を、src/bedrock_agentcore/memory/client.py の MemoryClient.__init__() と比較します。編集する前に、提案されている3つの選択肢を確認し、意図された非推奨化ポリシーを maintainer と確認します。ドキュメントと承認された警告動作が MemoryClient の状態を一貫して説明していれば完了です。

索引モデルが issue の本文から書いたものです。