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.

该 issue 未指明实现文件、测试或入口点；请先定位 CLI 现有的图像读取能力以及基于用量的计费集成。明确图像生成请求的调用、计费和写入项目目录的方式，然后验证图标、favicon、logo 和 OG 图像等开发者资源是否会生成到请求的路径。