fix: migrate from NPM_CONFIG_PRODUCTION to NPM_CONFIG_OMIT

[iloveitaly]

Replace the deprecated NPM_CONFIG_PRODUCTION environment variable with
explicit npm dependency-group flags:

• npm install and npm ci use --include=dev so build dependencies remain
  available while NODE_ENV=production.
• npm prune uses --omit=dev, matching the previous production-pruning
  behavior while retaining optional dependencies.
• The deprecated variable is no longer included in the runtime image.

Breaking Changes

• Runtime npm commands now follow NODE_ENV=production. An npm install run
  inside a deployed image will omit dev dependencies instead of inheriting
  NPM_CONFIG_PRODUCTION=false. Applications or scripts that inspect that
  variable will no longer see it.
• The generated npm install command explicitly uses --include=dev. npm gives
  include precedence over omit, so an existing .npmrc containing
  omit=dev will no longer suppress dev dependencies during Railpack's install
  step. Users who intentionally need that behavior can replace the install
  command, for example with
  RAILPACK_INSTALL_CMD="npm ci --omit=dev".
• --include=dev is supported by npm 7 and newer. Projects pinned to npm 6 or
  earlier may not install dev dependencies correctly while
  NODE_ENV=production and should override the install command or upgrade npm.
• Install lifecycle scripts and custom prune commands that explicitly depend
  on NPM_CONFIG_PRODUCTION may behave differently. NODE_ENV remains set to
  production.
• Generated build plans now contain npm ci --include=dev or
  npm install --include=dev. Tooling that matches exact command strings may
  require an update, and existing build caches will be invalidated once.

Optional dependencies continue to be installed, matching the behavior before
this migration.