Add MiniMax provider to the AI provider registry

[octo-patch]

Reason: Add MiniMax provider to the AI provider registry, auto-detection cascade, and model configuration\n\n## Changes\n\n- Add minimax-provider.ts implementing the AIProvider interface against the MiniMax Anthropic-compatible endpoint (default model MiniMax-M3, default base URL https://api.minimax.io/anthropic, China region https://api.minimaxi.com/anthropic)\n- Register MiniMax in provider-factory.ts: import, ProviderName type, PROVIDER_CREATORS map, and the auto-detection cascade (MINIMAX_API_KEY)\n- Update the provider enumeration in setup.ts (VALID_PROVIDERS, help text, validateProvider, interactive choices)\n- Document MiniMax in the providers README.md (table, architecture diagram, auto-detection list), root README.md, .env.example, hooks .env.example, CONFIG.md, CLAUDE_INTEGRATION_GUIDE.md, and the skill-activation-prompt.ts header comment\n\n## Verification\n\n- npx tsc --noEmit in .claude/hooks passes with no errors

[octo-patch]

Updated the MiniMax provider configuration to list both MiniMax-M3 and MiniMax-M2.7 and document the global and China Anthropic-compatible endpoints. I ran npm run check, a constants assertion, and the setup help smoke test.