Integration of xAI Grok as a new LLM provider in Crucix

## Summary
Add xAI Grok as a supported LLM provider in Crucix.

Crucix already supports multiple providers (`anthropic`, `openai`, `gemini`, `codex`, `openrouter`, `minimax`, `mistral`). Grok would be a useful addition for contributors and operators who already use xAI models and want another option for briefing synthesis, alert evaluation, and idea generation.

## Proposed scope
- add `grok` / xAI provider wiring to the LLM abstraction
- support environment-based configuration in the same style as existing providers
- define a sensible default model
- document setup in `README.md` / env docs
- add provider tests comparable to the other LLM integrations

## Expected implementation areas
- provider selection / dispatch
- request formatting and auth headers
- model default and model override handling
- any provider-specific response parsing
- tests for normal request/response behavior
- optional env-gated integration test if the repo pattern already supports that

## Acceptance criteria
- `LLM_PROVIDER=grok` works for the same flows as other providers
- failures degrade gracefully and do not crash sweeps
- docs explain required env vars and model defaults
- tests pass and match the repo?s current LLM provider patterns

## Notes
Please follow the existing provider implementation style instead of introducing a one-off path. If someone in the community wants to pick this up, comment on the issue and go for it.