feat: add Grok/XAI support to the LLM abstraction

[bhouston]

Add Grok/XAI Support to the LLM Abstraction

This PR adds support for Grok/XAI models to the LLM abstraction, as requested in issue #148.

Changes

• Created a new XAI provider implementation that follows the same pattern as the Anthropic provider
• Updated the provider registry to include the XAI provider
• Updated configuration files to support XAI models and API keys
• Added tests for the XAI provider
• Added documentation for using Grok/XAI models with MyCoder

Testing

The implementation has been tested with:

• Unit tests for the XAI provider
• Manual verification of the configuration changes

Documentation

Added a new documentation file docs/grok-integration.md that explains how to:

• Configure the XAI API key
• Set XAI as the default provider
• Select different Grok models
• Use Grok for a single command
• Troubleshoot common issues

Notes

• The implementation follows the OpenAI-compatible API format used by XAI/Grok
• Tool calls are fully supported, following the same pattern as other providers
• API key configuration works through both environment variables and the config system

Closes #148