Add MiniMax video generation skill

[octo-patch]

Reason: Bundled Seedance T2V/I2V tooling is shipped, but no MiniMax video endpoint, model/schema adapter, tests, or registry entry exists, and no related PR covers it.

Adds a focused built-in skill for text-to-video generation with:

• global and China endpoints
• the current v2 model, request schema, and duration validation
• task polling and completed video download handling
• a default registry entry

Checks:

• node --check SKILLs/minimax-video/scripts/generate_video.js
• jq -e . SKILLs/skills.config.json
• git diff --check

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