feat: --layout monorepo (pragmatic-layered, pnpm workspaces) for FE+BE

[spencer-osbrjp]

Context

Org practice is DDD + monorepo whenever FE and BE ship together. Add a --layout monorepo (default flat) that, when a backend is selected, emits a pnpm-workspace monorepo instead of a colocated single package.

Chosen shape (pragmatic layered)

<name>/
  package.json          # private workspace root: scripts + packageManager
  pnpm-workspace.yaml    # apps/*, packages/*
  apps/
    web/                 # the current frontend scaffold + its package.json
    api/                 # backend (hono/elysia + orm/db) + its package.json  [only if --backend]
  packages/
    domain/              # shared contract: zod schemas + types (workspace:* dep)

• Tooling: pnpm workspaces (matches default PM).
• packages/domain is the shared kernel; when --validation zod it ships an example schema, otherwise plain TS types. Both apps depend on it via workspace:*.

Scope

• pkg/config.go: Layout enum + field (flat|monorepo), default flat
• cmd/create.go: --layout flag + validation
• pkg/scaffold.go: layout-aware roots — FE → apps/web, backend/orm → apps/api, plus root workspace files + packages/domain
• pkg/packagejson.go: split package.json generation (web / api / domain / root); FE build skips backend/orm deps in monorepo mode
• config/templates/monorepo/*: root package.json, pnpm-workspace.yaml, domain package, api tsconfig
• Tests

Notes / out of scope

• Depends on the backend + orm/db work in PR feat: add backend framework and ORM/DB add-on categories #80; implemented on that branch since the auto-PR bot cuts from main.
• Turborepo/Nx, root-hoisted shared configs, and auto-defaulting layout to monorepo when --backend is set — deferred.