feat: add Atlas Cloud image generation skill

[binyangzhu000-sudo]

Summary

• add an atlascloud-img-gen OpenClaw skill for text-to-image and image editing
• support local reference-image upload, asynchronous prediction polling, and output downloads
• keep credentials client-side through ATLASCLOUD_API_KEY and use only Python standard-library dependencies

Verification

• python3 -m unittest discover -s skills/atlascloud-img-gen/scripts/tests -v (5 tests)
• python3 -m compileall -q skills/atlascloud-img-gen/scripts
• bash -n skills/atlascloud-img-gen/atlascloud-img-gen.sh
• live Atlas Cloud text-to-image submission, polling, and image download
• git diff --check

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