Add MiniMax image still generation support

[octo-patch]

Reason: Add MiniMax image-01 still generation support for the scroll-world pipeline.

Changes:

• Added a MiniMax image helper for region-aware /v1/image_generation requests.
• Documented MiniMax endpoint selection, request fields, output formats, and response parsing.
• Added a still-generation pipeline snippet and intake option for MiniMax stills.

Checks:

• PYTHONDONTWRITEBYTECODE=1 python3 -m py_compile skills/scroll-world/references/minimax_image.py
• PYTHONDONTWRITEBYTECODE=1 python3 skills/scroll-world/references/minimax_image.py --help