Add NixOS server quick-start guide

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server-quickstart.md

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+# NixOS server quick-start
+
+Get a new NixOS machine online as a server: hostname, user, SSH, and key-based login.
+
+## 1. Prerequisites
+
+- NixOS installed
+- Machine on the network
+- Console or existing SSH access
+
+## 2. Hostname
+
+In your config (e.g. `/etc/nixos/configuration.nix` or a flake module):
+
+```nix
+networking.hostName = "myserver";
+```
+
+## 3. User
+
+Define a user in `users.users` (e.g. `danny` or `admin`). For key-only login you can leave the password empty or set a hashed one.
+
+```nix
+users.users.danny = {
+  isNormalUser = true;
+  extraGroups = [ "wheel" "networkmanager" ];
+  # Optional: hashed password, or omit for key-only login
+  # hashedPassword = "...";  # mkpasswd -m sha-512
+};
+```
+
+## 4. SSH
+
+Enable OpenSSH and add your public keys so you can log in without a password.
+
+```nix
+services.openssh.enable = true;
+
+users.users.danny.openssh.authorizedKeys.keys = [
+  "ssh-ed25519 AAAA... your-key-comment"
+  # Add more keys (e.g. from ~/.ssh/id_*.pub on your client)
+];
+```
+
+Optional hardening (disable password auth, restrict root):
+
+```nix
+services.openssh.settings = {
+  PasswordAuthentication = false;
+  PermitRootLogin = "no";
+};
+```
+
+## 5. Apply and test
+
+Rebuild and switch:
+
+```bash
+sudo nixos-rebuild switch
+```
+
+If you use a flake:
+
+```bash
+sudo nixos-rebuild switch --flake /path/to/dotfiles/nixos#hostname
+```
+
+From your main machine, test:
+
+```bash
+ssh danny@myserver
+```
+
+Replace `danny` and `myserver` with your user and hostname.