docs: add Marketplace README for VS Code extension package

[ktmage]

Summary / 概要

Adds a dedicated README.md inside packages/platforms/vscode/ so that the VS Code Marketplace store page displays the extension description correctly.

VS Code Marketplace のストアページに拡張機能の説明が正しく表示されるよう、packages/platforms/vscode/ に専用の README.md を追加します。

Background / 背景

After the monorepo migration, README.md remained only at the repository root. Since vsce package packages the README.md from the extension directory (where package.json lives), the .vsix file no longer included a README, causing the Marketplace page to show no description.

モノレポ移行後、README.md はリポジトリルートにしか存在しませんでした。vsce package は拡張機能ディレクトリの README.md をパッケージに含めるため、.vsix に README が含まれず、Marketplace ページの説明が空になっていました。

Changes / 変更内容

• New file: packages/platforms/vscode/README.md
  • User-facing content extracted from the root README (features, requirements, installation, demo)
  • Development-specific sections (build, project structure) excluded
  • Links to CONTRIBUTING.md and LICENSE use absolute GitHub URLs (relative paths do not work on Marketplace)
  • English and Japanese bilingual sections maintained

Testing / テスト

• Verified the file is not excluded by .vscodeignore
• Content renders correctly as Markdown