feat: add Kenya Law RAG and trusted web search tools

Implements two retrieval tools that ground Iroh's legal responses in
authoritative Kenyan sources rather than training-data recall:

- search_kenya_law: ChromaDB semantic search over the Kenya Law corpus
  using Xenova/all-MiniLM-L6-v2 embeddings (384-dim, mean pool,
  normalized). Lazy-cached embedder and collection. east_africa
  jurisdiction filter applied via where clause; kenya filter omitted
  to avoid silently dropping chunks that predate the metadata field.
  Returns 400-char snippets with distance; notes weak results (>0.6).

- search_legal_authority_web: Tavily search restricted to 12 trusted
  Kenyan/East African legal authority domains (kenyalaw.org,
  parliament.go.ke, judiciary.go.ke, etc.). Never throws - all
  failures surface as { results: [], note }.

Both tools registered in TOOLS[] and dispatched in runToolCalls.
chromadb and @xenova/transformers added as dependencies.
CHROMA_DB_PATH and TAVILY_API_KEY documented in .env.example.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>