feat: add optional Atlas Cloud image provider

[binyangzhu000-sudo]

Summary

• add an explicit --provider atlas path while keeping Gemini as the default
• submit Atlas Cloud Nano Banana text-to-image jobs once and poll predictions with bounded timing
• document provider-specific credentials, model defaults, options, and current text-to-image limit
• add focused tests for endpoint selection, async polling, and no-retry submission behavior

Testing

• python3 -m unittest skills/nanobanana-skill/test_nanobanana.py -v
• python3 -m py_compile skills/nanobanana-skill/nanobanana.py skills/nanobanana-skill/test_nanobanana.py
• python3 -m scripts.quick_validate ../nanobanana-skill from skills/skill-creator
• uvx ruff check skills/nanobanana-skill/nanobanana.py skills/nanobanana-skill/test_nanobanana.py
• uvx ruff format --check skills/nanobanana-skill/nanobanana.py skills/nanobanana-skill/test_nanobanana.py

The live Atlas model catalog and input schema were checked before implementation. One live generation submit returned HTTP 402 before creating a prediction and was not retried.