fix(skills): retire unavailable MiniMax music workflows

[dvd233]

Summary

• mark minimax-music-gen and buddy-sings as retired compatibility guards
• remove instructions that install or invoke the removed MiniMax Music CLI/SDK commands
• synchronize the English and Chinese catalog entries

Why

Fixes #115. MiniMax-AI/cli#239 removed the Music CLI/SDK surface while the current skills still direct agents to mmx music generate / mmx music cover. A fresh installation therefore follows a guaranteed unavailable workflow.

Scope

minimax-music-playlist is intentionally not changed because open PR #78 already modifies that file. minimax-multimodal-toolkit is also intentionally excluded because open PRs #57/#103/#105 touch that skill. Those remain separate follow-ups.

Validation

• custom frontmatter, catalog, and retired-command assertions pass
• git diff --check passes
• changed-file scan contains no retired Music commands or install instructions
• repository validator was run; its only error is the pre-existing minimax-multimodal-toolkit frontmatter name mismatch (the file is outside this PR and is covered by existing PRs feat(minimax-multimodal-toolkit): update models, simplify scripts, and standardize docs #57/fix: fix minimax-multimodal-toolkit wrong frontmatter name #105)

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}