Brave's /res/v1/llm/context returns pre-extracted text chunks
optimized for LLM consumption instead of URLs + short snippets.
Enable with BRAVE_LLM_CONTEXT=1 env var; same API key and pricing.
- Add _search_llm_context() and _normalize_llm_context() to brave_search.py
- Wire opt-in flag through _search_web() in last30days.py
- Update module docstring (free tier eliminated Feb 2026)
- Add 23 tests covering normalization, filtering, date parsing