refactor(car): separate frontend and backend tool surfaces

[x-lixu]

Summary

• organize cockpit capabilities as five domain-oriented tool groups with colocated MCP manifests, executors, and sample catalogs
• expose weather through the foreground MCP profile while keeping the other 14 tools on a backend-only MCP surface
• remove weather from the cockpit A2A router and spawn_thinking capability description
• keep both tool surfaces on the same authoritative cockpit state and add startup readiness protection
• document the extension point and allow Frontend MCP URLs to reference one environment variable

Validation

• npm run test:car
• frontend MCP config/client/profile tests
• npm run lint
• npm run example:car:build
• isolated four-process smoke test; Gateway reported one ready frontend MCP tool