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/

.gitignore

@@ -0,0 +1,8 @@
+.DS_Store
+*.swp
+*.bak
+result
+result-*
+.direnv/
+*.local.nix
+nh-output/