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/

hosts/common/default.nix

@@ -0,0 +1,18 @@
+{ pkgs, ... }:
+{
+  # flake + nix-command
+  nix.settings.experimental-features = [
+    "nix-command"
+    "flakes"
+  ];
+
+  users.users.mikl = {
+    isNormalUser = true;
+    shell = pkgs.zsh;
+    home = "/home/mikl";
+  };
+
+  environment.systemPackages = [
+    pkgs.vim
+  ];
+}