Add archival notice to README

[Copilot]

This repository is being archived as development has migrated to microsoft/vscode-containers.

Changes

• Prepends an archival notice block to README.md directing contributors to the new 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This repository has been archived.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.

[Copilot]

Pull request overview

Adds a prominent archival notice to the top of the root README.md, directing contributors to the active development repository (microsoft/vscode-containers) now that this repo is being archived.

Changes:

• Prepends an “⚠️ Archived” header and a blockquoted notice linking to microsoft/vscode-containers.
• Leaves the remainder of the README content unchanged.

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