refactor: add resolved agreement negotiation core

[tobytkershaw]

Summary

• add ResolvedAgreement as the canonical internal negotiation output for named-offer and bespoke pre-contract negotiation
• compile negotiated agreements through one deterministic ResolvedAgreement -> RelayContract seam while preserving the existing external negotiated_contract output shape
• migrate relay handles to store resolvedAgreement canonically and cover the new path with focused unit and AFAL regression tests

Testing

• npm run build in packages/agentvault-client
• npm run build in packages/agentvault-mcp-server
• npm test -- --run src/__tests__/resolved-agreement.test.ts src/__tests__/contract-negotiation.test.ts src/__tests__/relaySignal-afal.test.ts in packages/agentvault-mcp-server