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;

lib/llm/minimax.mjs

@@ -0,0 +1,51 @@
+// MiniMax Provider — raw fetch, no SDK
+// Uses MiniMax's OpenAI-compatible Chat Completions API
+
+import { LLMProvider } from './provider.mjs';
+
+export class MiniMaxProvider extends LLMProvider {
+  constructor(config) {
+    super(config);
+    this.name = 'minimax';
+    this.apiKey = config.apiKey;
+    this.model = config.model || 'MiniMax-M2.5';
+  }
+
+  get isConfigured() { return !!this.apiKey; }
+
+  async complete(systemPrompt, userMessage, opts = {}) {
+    const res = await fetch('https://api.minimax.io/v1/chat/completions', {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+        'Authorization': `Bearer ${this.apiKey}`,
+      },
+      body: JSON.stringify({
+        model: this.model,
+        max_tokens: opts.maxTokens || 4096,
+        messages: [
+          { role: 'system', content: systemPrompt },
+          { role: 'user', content: userMessage },
+        ],
+      }),
+      signal: AbortSignal.timeout(opts.timeout || 60000),
+    });
+
+    if (!res.ok) {
+      const err = await res.text().catch(() => '');
+      throw new Error(`MiniMax API ${res.status}: ${err.substring(0, 200)}`);
+    }
+
+    const data = await res.json();
+    const text = data.choices?.[0]?.message?.content || '';
+
+    return {
+      text,
+      usage: {
+        inputTokens: data.usage?.prompt_tokens || 0,
+        outputTokens: data.usage?.completion_tokens || 0,
+      },
+      model: data.model || this.model,
+    };
+  }
+}