feat: add optional Atlas Cloud image generation

[binyangzhu000-sudo]

Summary

• add an optional Atlas Cloud helper for marketing image generation
• discover visible image models and validate the selected model against its live schema before submission
• submit generation once, use bounded result polling, and optionally download the generated image
• document the opt-in workflow without changing the skill's native-tool default

Validation

• python3 -m unittest discover -s skills/marketing-image-generation/scripts -p 'test_*.py' -v
• python3 .agents/skills/skill-creator/scripts/quick_validate.py skills/marketing-image-generation
• python3 skills/marketing-image-generation/scripts/atlas_image.py list --search gpt-image-2
• one real generation smoke produced a valid 1024x1024 PNG
• git diff --check

The implementation was prepared with AI assistance and manually reviewed and tested.

---

Summary by cubic

Adds an optional Atlas Cloud provider for marketing image generation, used only when the user explicitly requests it or no native image tool is available. The skill's native tool default remains unchanged.

• Adds atlas_image.py with commands to list visible image models and generate images.
• Validates the selected model and its live input schema before submission, passing only schema-supported options.
• Submits exactly one generation request, polls with a bounded loop, and prints the output URL or downloads it with --output.
• Documents the setup and usage in SKILL.md and adds unit tests for the helper.

^{Written for commit dc64307. Summary will update on new commits.}

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}