feat(llm): add Grok (xAI) integration (#179)

[david-mamani]

Description

Closes #179 (2026 Model Provider Extensions - Grok)

This PR implements full support for Grok (xAI) as an LLM provider, compliant with the strict CI/CD requirements (Python 3.13, uv locked dependencies, and Typed Python).

Changes

• Feat: Added GrokBackend in src/memu/llm/backends/grok.py (inherits from OpenAI base).
• Config: Updated settings.py to support provider="grok" (Defaults to https://api.x.ai/v1).
• Deps: Added openai to pyproject.toml and updated uv.lock.
• Docs: Added setup guide in docs/providers/grok.md.
• Tests: Added tests/llm/test_grok_provider.py with unittest.mock to simulate API responses without network calls.

Verification

• uv run pytest passed (3 tests).
• uv run ruff check passed.
• uv sync ensures lockfile integrity.

Checklist

• My code follows the code style of this project (ruff formatted).
• I have updated the documentation accordingly.
• I have added tests that prove my fix is effective.

resolves #179

[ankaisen]

@david-mamani Thank you! LGTM 👍