Add worked example: server-host and device-host flakes

Self-contained example under examples/ with full NixOS flakes for both
sides of a deployment (control server + binary cache vs. an agent
device), plus a README walking through the end-to-end install + first
publish.

examples/server-host/flake.nix

@@ -0,0 +1,19 @@
+{
+  description = "ota.example.com — nix-ota control plane + binary cache";
+
+  inputs = {
+    nixpkgs.url  = "github:nixos/nixpkgs?ref=nixos-unstable";
+    nix-ota.url  = "git+https://linus.dyrehytten.dk/max/nix-ota";
+    nix-ota.inputs.nixpkgs.follows = "nixpkgs";
+  };
+
+  outputs = { self, nixpkgs, nix-ota, ... }: {
+    nixosConfigurations.ota = nixpkgs.lib.nixosSystem {
+      system = "x86_64-linux";
+      modules = [
+        nix-ota.nixosModules.server
+        ./configuration.nix
+      ];
+    };
+  };
+}