Add archival notice to root README

[Copilot]

This repository is being archived, and contributors should be redirected to microsoft/vscode-containers for future issues and pull requests. This change adds that notice at the top of the root README without modifying any other README content or any other files.

• README archival notice

  • Inserts the required archived banner as the first block in README.md
  • Points contributors to microsoft/vscode-containers for ongoing development and contribution flow

• Scope

  • Leaves the remainder of the README unchanged
  • Limits the update to the repository root README.md only

# ⚠️ Archived

> **This repository has been archived.** Development has moved to [microsoft/vscode-containers](https://github.com/microsoft/vscode-containers). Please open new issues and pull requests in that repository.

Original prompt

This repository is being archived because its code has been migrated to microsoft/vscode-containers. Update the root README file (the README.md at the repository root) to add an archival notice at the very top, before all other content.

Insert exactly the following Markdown block, verbatim, as the first lines of the file, followed by a blank line before the existing content:

⚠️ Archived

This repository has been archived. Development has moved to microsoft/vscode-containers. Please open new issues and pull requests in that repository.

Requirements:

• Do not modify, reword, or reformat any other part of the README.
• Do not change any other files in the repository.
• Keep the exact wording, emoji (⚠️), heading, and link shown above.

In the PR description, briefly note that this adds an archival notice to the README pointing contributors to microsoft/vscode-containers.