Support image generation for developer assets (icons, favicons, logos, OG images)

Describe the feature or problem you'd like to solve

Copilot CLI can read images but cannot produce them. Building any web page, desktop app,
or mobile app needs visual assets that aren't code — app icons at every required size,
favicon.ico, Open Graph preview images, splash screens, placeholder art.

Copilot CLI can scaffold the whole project and write the manifest that references
icon-512.png, but it can't create that file. The developer has to leave the terminal,
generate the asset elsewhere, and come back. That happens on nearly every new project.

To be clear on direction: the deliverable is a file written into the project
directory, not an image displayed in the terminal.

Why now — billing is usage-based. Since 1 June 2026 Copilot meters usage through AI
Credits (1 credit = $0.01). Image generation is a paid capability and I'd expect to pay
for it per use, charged at its own rate rather than the one designed for text.

Additional context

OpenAI's Codex CLI already ships this as a bundled first-party skill (imagegen) backed
by a built-in image_gen tool, scoped to "website assets, game assets, UI mockups,
product mockups, wireframes, logo design," with a policy that project-bound assets get
written into the workspace rather than left in the tool's default directory.

Also requested for VS Code:
microsoft/vscode#266055.

この issue では実装ファイル、テスト、エントリーポイントが指定されていないため、まず CLI に既存の画像読み取り機能と、使用量ベースの課金統合がどこにあるかを特定します。画像生成リクエストがどのように呼び出され、課金され、プロジェクトディレクトリに書き込まれるかを定義し、そのうえで、アイコン、favicon、ロゴ、OG 画像などの開発者向けアセットが要求されたパスに生成されることを確認します。