Replace the project directory tree and PR integration/versioning
sections with a new "Philosophie de développement" section covering
the NASA Power of Ten rules and security-by-design principles.
- Remove `zen-setup` binary from package.json and package-lock.json
- Replace manual setup steps with `npx @zen/start` CLI command
- Simplify INSTALL.md by reducing steps from 6 to 4
- Update DEV.md to reflect removal of zen-setup from CLI scripts
- Fix relative path to `.env.example` in documentation