refactor: merge setup.sh into update.sh for seamless first-run

- update.sh now auto-generates .env from .env.example if missing
- Generates random JWT/SALT keys on first run
- Existing .env is never overwritten
- Deleted redundant setup.sh
- Updated all documentation references

AGENTS.md

@@ -28,8 +28,7 @@ NexusGuard SD-WAN Suite: Enterprise Zero-Trust SD-WAN with WireGuard tunneling,
 ├── docker-compose.yml    # Production orchestration
 ├── docker-compose.dev.yml# Dev (air hot-reload)
 ├── Makefile              # up/down/dev/migrate/reset-db
-├── setup.sh              # First-run: generate .env + random keys
-├── update.sh             # Docker update: pull/build/migrate
+├── update.sh             # Docker update: auto-generate .env + pull/build/migrate
 ├── nexusguard-install.sh # Native install (systemd + nginx)
 ├── nexusguard-uninstall.sh
 ├── .env.example          # DB/JWT/SALT/VITE config template