feat(skills): add MiniMax-AI/cli as default skill tap

[octo-patch]

Summary

• Add MiniMax-AI/cli to the Skills section of README.md so the mmx-cli skill is discoverable out of the box
• Users can install via install_skill with source: "git", url: "https://github.com/MiniMax-AI/cli", name: "mmx-cli" — the SKILL.md is fetched directly from MiniMax-AI/cli
• Skill updates are fully decoupled: MiniMax maintains the upstream SKILL.md, no changes needed in this project

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What is mmx-cli?

mmx-cli is a CLI tool for the MiniMax AI platform, providing:

• Text generation (MiniMax-M2.7 model)
• Image generation (image-01)
• Video generation (Hailuo-2.3)
• Speech synthesis (speech-2.8-hd, 300+ voices)
• Music generation (music-2.6, with lyrics, cover, and instrumental)
• Web search

The SKILL.md follows the agentskills.io standard and includes agent-specific flags (--non-interactive, --quiet, --output json).

Test plan

• Automaton agent running install_skill with the provided git source successfully installs mmx-cli
• list_skills shows the mmx-cli skill after installation
• Ask agent "generate a jazz instrumental" — agent invokes mmx music generate via terminal