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.

No files or tests are named. Start by reviewing the app's existing built-in Copilot tools and how they access other Copilot surfaces, then determine how memories can be retrieved without a public REST endpoint. Done means chat can read and display a user's Copilot memories; management is an additional goal stated in the request.

Written by the indexing model from the issue text.