Copilot memories not accessible in the GitHub Copilot app chat

## Problem

Copilot memories (managed at `github.com/settings/copilot/memory`) are surfaced in other Copilot surfaces (VS Code, CLI) but the Copilot app chat agent has no tool/integration to retrieve or display them. There is no public REST API endpoint for memories, so the agent falls back to `gh api` calls that return 404.

## Request

Add a built-in tool in the app so the agent can read (and ideally manage) a user's Copilot memories within the chat interface — consistent with other Copilot surfaces.

未指定任何文件或测试。首先查看应用现有的内置 Copilot 工具，以及它们如何访问其他 Copilot 界面，然后确定如何在没有公共 REST 端点的情况下检索记忆。完成的标准是聊天可以读取并显示用户的 Copilot 记忆；管理是请求中提出的另一项目标。

由索引模型根据 Issue 内容生成。