feat(image-gen): add Atlas Cloud provider

[binyangzhu000-sudo]

Summary

• add Atlas Cloud as an optional baoyu-image-gen provider and workspace setup choice
• map image options to the live Atlas Cloud model schema and poll asynchronous predictions with a bounded wait
• submit each Atlas Cloud generation task once without POST retries
• document configuration and cover provider selection, request mapping, and secret setup with tests

Validation

• npm run build
• npm test
• npx -y bun test skills/baoyu-image-gen/scripts/main.test.ts skills/baoyu-image-gen/scripts/providers/atlascloud.test.ts
• live 1K text-to-image smoke with google/nano-banana-2/text-to-image (one generation attempt)