feat: add MiniMax as LLM provider

Add MiniMax (api.minimax.io) as a fifth LLM provider option alongside
Anthropic, OpenAI, Gemini, and Codex. MiniMax offers an
OpenAI-compatible Chat Completions API with the M2.5 model (204K
context window).

Changes:
- lib/llm/minimax.mjs: new provider using raw fetch (no SDK)
- lib/llm/index.mjs: register MiniMax in the factory
- .env.example, crucix.config.mjs, README.md: document the new option
- test/llm-minimax.test.mjs: 10 unit tests (node:test)
- test/llm-minimax-integration.test.mjs: live API integration test

Usage:
  LLM_PROVIDER=minimax
  LLM_API_KEY=sk-...
  LLM_MODEL=MiniMax-M2.5          # optional, this is the default

lib/llm/index.mjs

@@ -4,12 +4,14 @@ import { AnthropicProvider } from './anthropic.mjs';
 import { OpenAIProvider } from './openai.mjs';
 import { GeminiProvider } from './gemini.mjs';
 import { CodexProvider } from './codex.mjs';
+import { MiniMaxProvider } from './minimax.mjs';
 
 export { LLMProvider } from './provider.mjs';
 export { AnthropicProvider } from './anthropic.mjs';
 export { OpenAIProvider } from './openai.mjs';
 export { GeminiProvider } from './gemini.mjs';
 export { CodexProvider } from './codex.mjs';
+export { MiniMaxProvider } from './minimax.mjs';
 
 /**
  * Create an LLM provider based on config.
@@ -30,6 +32,8 @@ export function createLLMProvider(llmConfig) {
       return new GeminiProvider({ apiKey, model });
     case 'codex':
       return new CodexProvider({ model });
+    case 'minimax':
+      return new MiniMaxProvider({ apiKey, model });
     default:
       console.warn(`[LLM] Unknown provider "${provider}". LLM features disabled.`);
       return null;