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

crucix.config.mjs

@@ -7,7 +7,7 @@ export default {
   refreshIntervalMinutes: parseInt(process.env.REFRESH_INTERVAL_MINUTES) || 15,
 
   llm: {
-    provider: process.env.LLM_PROVIDER || null, // anthropic | openai | gemini | codex
+    provider: process.env.LLM_PROVIDER || null, // anthropic | openai | gemini | codex | minimax
     apiKey: process.env.LLM_API_KEY || null,
     model: process.env.LLM_MODEL || null,
   },