feat: restructure into Turborepo monorepo layout

Move plugin code (admin/, server/, jest.config.js) into
packages/strapi-plugin-boilerplate/ and playground into apps/playground/.

Add root workspace configuration:
- pnpm-workspace.yaml defining workspace packages
- turbo.json with task pipeline (build, dev, lint, test)
- Root package.json with workspace scripts

Update plugin package.json:
- Remove yalc from build script
- Remove playground and cypress scripts
- Add biome lint/format scripts
- Add @tanstack/react-query dependency
- Remove cypress, prettier devDeps
- Update ENV_PATH for new directory structure

Update playground to use workspace:* protocol instead of yalc link.

Author: ayhid