feat: vercelJson genie generator

Summary

Provide a vercelJson() genie helper in effect-utils that generates vercel.json with the correct megarepo-aware installCommand baked in.

Motivation

Consumer repos currently maintain vercel.json files manually with a boilerplate installCommand pattern:

{
  "installCommand": "cd ../.. && bash scripts/vercel-install.sh && cd - && pnpm install --no-frozen-lockfile"
}

This pattern is the same across all packages that deploy to Vercel and could be generated.

Proposed API

// packages/app/vercel.json.genie.ts
import { vercelJson } from '../../repos/effect-utils/genie/external.ts'

export default vercelJson({
  framework: 'nextjs',
  buildCommand: 'pnpm build',
  outputDirectory: '.next',
})

The generator would:

• Auto-generate the installCommand with the cd ../.. / cd - dance relative to the package location
• Accept standard Vercel config fields (framework, buildCommand, outputDirectory, rewrites, etc.) as passthrough
• Reference the consumer's install script (e.g. scripts/vercel-install.sh) in the generated command

Optional: install script generator

A megarepoInstallScript() genie helper that generates the CI install script from megarepo.lock:

// scripts/vercel-install.sh.genie.ts
import { megarepoInstallScript } from '../repos/effect-utils/genie/external.ts'
export default megarepoInstallScript()

This might be better kept in user land since:

• The script is simple and stable (rarely changes)
• Each repo may have specific cloning needs or extra setup steps
• Keeping it in the consumer gives more flexibility without much maintenance cost

Context

• CI deployment pattern docs

Start by reading the existing genie helpers in repos/effect-utils/genie/external.ts and the proposed packages/app/vercel.json.genie.ts usage. Compare the requested Vercel passthrough fields and installCommand path behavior with the CI deployment pattern documented in pull request 170. Done means a vercelJson() helper can generate the megarepo-aware vercel.json configuration; assess the optional scripts/vercel-install.sh.genie.ts separately.