Add project scaffolding command for new packages

[gtbuchanan]

Summary

`turbo:init` generates turbo.json and scripts for existing projects
by discovering capabilities from deps and config files. New packages
start empty — there's no way to scaffold a package with the right
configs, deps, and directory structure from scratch.

Use cases

• Adding a new package to an existing monorepo
• Bootstrapping a new consumer repo with tooling presets
• Creating a package with specific capabilities (TypeScript, ESLint,
  Vitest, etc.) without manually wiring everything

Possible approaches

• Interactive CLI: `gtb create` prompts for package name, capabilities
• Template-based: `gtb create --template lib` with preset templates
• Flag-based: `gtb create --typescript --eslint --vitest`

Context

Discussed during #17 (Turborepo integration). `turbo:init` handles
the "make existing project work with turbo" case. This covers the
"start a new project from scratch" case.