Add xAI Grok as an LLM provider

## 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.