`ajv` is required at runtime by the installer (`scripts/lib/install/config.js`)
but was listed under `devDependencies`. This caused `Error: Cannot find module
'ajv'` when running `./install.sh` from a fresh git clone or via `npx`.
- Move `ajv` from devDependencies to dependencies in package.json
- Add auto `npm install` in install.sh when node_modules is missing
- Add matching auto-install in install.ps1 for Windows parity
