docs: remove zen-setup CLI and simplify installation guide

- 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

package-lock.json

@@ -21,8 +21,7 @@
         "stripe": "^14.0.0"
       },
       "bin": {
-        "zen-db": "dist/cli/database.js",
-        "zen-setup": "dist/cli/setup.js"
+        "zen-db": "dist/cli/database.js"
       },
       "devDependencies": {
         "@tailwindcss/cli": "^4.2.1",