feat: negotiate ambiguous demo purposes

[tobytkershaw]

Summary

• add acceptable_purposes to agentvault.relay_signal so direct AFAL flows can negotiate a common contract offer without an early single-purpose lock-in
• use that capability in the demo runtime via structured scenario metadata instead of prompt wording tweaks
• mark the salary negotiation scenario as an ambiguous overlap case and guard it with tests

Testing

• cd /Users/tobykershaw/Development/vcav-io/av-codex-purpose-negotiation/packages/agentvault-client && npm run build
• cd /Users/tobykershaw/Development/vcav-io/av-codex-purpose-negotiation/packages/agentvault-mcp-server && npm run build
• cd /Users/tobykershaw/Development/vcav-io/av-codex-purpose-negotiation/packages/agentvault-mcp-server && npm test -- --run src/__tests__/relaySignal-afal.test.ts
• cd /Users/tobykershaw/Development/vcav-io/av-codex-purpose-negotiation/packages/agentvault-demo-ui && npm run build
• cd /Users/tobykershaw/Development/vcav-io/av-codex-purpose-negotiation/packages/agentvault-demo-ui && npm test -- --run src/scenario-purpose-registry.test.ts src/guardrails.test.ts

Closes #329