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/

README.md

@@ -0,0 +1,15 @@
+# nix-config
+
+Nix configurations for iscg infra hosts.
+
+Hosts and naming scheme: see [garden.md](./garden.md).
+
+## rebuild
+
+```bash
+# poppy (macOS)
+darwin-rebuild switch --flake .#poppy
+
+# muscari, rosemary (NixOS)
+sudo nixos-rebuild switch --flake .#<host>
+```