initial scaffold: multi-host flake (poppy, muscari, rosemary)

- flake.nix with mkNixos + mkDarwin dispatchers
- hosts/common/{default.nix} (NixOS shared)
- hosts/common-darwin/{default.nix} (darwin shared)
- hosts/poppy/ (M1 MacBook Air, nix-darwin)
- hosts/muscari/ (k3s server, placeholder)
- hosts/rosemary/ (backup VM, placeholder)
- home/default.nix (home-manager for mikl)
- README.md, garden.md (host naming scheme: plants)
- secrets/.gitkeep

old muscari-only config moved to nix-config-legacy/

flake.nix

@@ -0,0 +1,77 @@
+{
+  description = "nix-config — multi-host Nix configurations";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";
+
+    nix-darwin = {
+      url = "github:LnL7/nix-darwin";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+
+    home-manager = {
+      url = "github:nix-community/home-manager";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+
+    sops-nix = {
+      url = "github:Mic92/sops-nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+
+    disko = {
+      url = "github:nix-community/disko";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs =
+    { self, nixpkgs, nix-darwin, home-manager, sops-nix, disko, ... }:
+    let
+      mkNixos = hostname:
+        nixpkgs.lib.nixosSystem {
+          system = "x86_64-linux";
+          modules = [
+            ./hosts/common
+            ./hosts/${hostname}
+            sops-nix.nixosModules.sops
+            disko.nixosModules.disko
+            home-manager.nixosModules.home-manager
+            {
+              home-manager = {
+                useGlobalPkgs = true;
+                useUserPackages = true;
+                users.mikl = import ./home;
+              };
+            }
+          ];
+        };
+
+      mkDarwin = hostname:
+        nix-darwin.lib.darwinSystem {
+          system = "aarch64-darwin";
+          modules = [
+            ./hosts/common-darwin
+            ./hosts/${hostname}
+            home-manager.darwinModules.home-manager
+            {
+              home-manager = {
+                useGlobalPkgs = true;
+                useUserPackages = true;
+                users.mikl = import ./home;
+              };
+            }
+          ];
+        };
+    in
+    {
+      nixosConfigurations = {
+        muscari = mkNixos "muscari";
+        rosemary = mkNixos "rosemary";
+      };
+
+      darwinConfigurations = {
+        poppy = mkDarwin "poppy";
+      };
+    };
+}