feat: add xAI Grok as LLM provider

[Karlblock]

Summary

Adds xAI Grok as the 8th LLM provider in Crucix.

• New lib/llm/xai.mjs provider using xAI's OpenAI-compatible Chat Completions API (https://api.x.ai/v1)
• Both xai and grok accepted as provider name (case-insensitive)
• Default model: grok-3
• Full unit test suite (test/llm-xai.test.mjs) — 10 tests, all passing
• Updated .env.example, README.md (provider table, architecture tree, env reference)

Usage

LLM_PROVIDER=xai
LLM_API_KEY=xai-your-key
LLM_MODEL=grok-3        # optional, this is the default

Test plan

• node --test test/llm-xai.test.mjs — 10/10 pass
• Factory creates XAIProvider for both xai and grok aliases
• Correct API endpoint, headers, and request body format
• Error handling and empty response edge cases
• Integration test with live xAI API key (not included, requires key)

Closes #54

[calesthio]

I haven't been able to get to this yet because work has been busy, but I definitely plan to review it over the weekend.

[3byss]

I have already taken up this issue as one of my first Github contributions.