Add MiniMax provider presets and API-key auto-detection

[octo-patch]

Reason: Add MiniMax provider presets and API-key auto-detection.

MiniMax was not among the built-in provider presets, so users had to set every MINIKODE_* variable by hand. This adds first-class MiniMax support so a single API key is enough to get started.

Changes:

• Add global (https://api.minimax.io/v1) and China (https://api.minimaxi.com/v1) MiniMax presets to API_PRESETS, defaulting to MiniMax-M3.
• Auto-detect MINIMAX_API_KEY; select the China endpoint with MINIMAX_REGION=cn and the model with MINIMAX_MODEL (defaults to MiniMax-M3; MiniMax-M2.7 is also available).
• Show "MiniMax" as the provider name in the LLM info display for MiniMax base URLs.
• Document the new environment variables in the README and docs/config.md.

Checks:

• pnpm exec tsc --noEmit
• pnpm exec vitest run — 265 tests pass, including new MiniMax auto-detection tests
• pnpm exec prettier --check on the changed files