Phase 0+A1: test infra, CopilotKit 1.55.2-next.1, single-source design system

- Add vitest (apps/app jsdom, apps/mcp node) + pytest (apps/agent) + turbo test task
- Pin @copilotkit/{react-core,react-ui,runtime} to 1.55.2-next.1 (canonical
  OpenGenerativeUI rails: generateSandboxedUi, OpenGenerativeUIMiddleware)
- New @repo/design-system package single-sources THEME_CSS / SVG_CLASSES_CSS /
  FORM_STYLES_CSS / importmap; apps/app and apps/mcp now consume it, killing
  the "keep in sync" fork in apps/mcp/src/renderer.ts (plan acceptance #4,
  enforced by a repo-scan test)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>