feat: add Kenyan legal authority grounding contract to system prompt

Replaces the open-ended "answer based on your legal knowledge" line with
an 8-rule honesty contract that:
- Requires retrieval via search_kenya_law (local corpus) and
  search_legal_authority_web (approved domains) before citing any
  Kenyan statute, case, regulation, or guidance
- Prohibits citing Kenyan legal authority from training data
- Requires surfacing corpus-vs-web conflicts and preferring newer authority
- Mandates an explicit "no results" admission when both tools come up empty
- Bans invented or paraphrased citations outright
- Flags EA Court of Appeal cases (pre-1977) as persuasive, not binding
- Directs users to kenyalaw.org / relevant regulator for out-of-corpus
  authorities

Retrieval tools (search_kenya_law, search_legal_authority_web) are not yet
registered - contract is placed ahead of implementation intentionally so
the model refuses rather than hallucinates if a retrieval gap occurs.

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