home-manager: enable for darman on every host, not just terra

terra was the only host with real ~/.zshrc/.zshenv (via home-manager),
so it never hit the plain-zsh zsh-newuser-install wizard that shows up
on first login everywhere else. Wire home-manager.nixosModules.home-manager
into jupiter/neptun/mars/mercury (+ mercury-vm/jupiter-vbox test targets)
and give darman terra's shared zsh baseline via home/common.nix, imported
from common.nix. terra's own home.nix now only carries its
desktop/dev-specific profile (Hyprland, alacritty, git identity, direnv,
dev packages) layered on top via home-manager.users.darman.imports.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Du1WQRk1F8DenrPhf8TofF

common.nix

@@ -53,6 +53,19 @@
 
   environment.systemPackages = with pkgs; [ git btop tmux curl wget zsh-powerlevel10k lsd ];
 
+  # ---- home-manager (user-level config for darman, all hosts) ----
+  # Requires home-manager.nixosModules.home-manager in the host's own
+  # `modules` list (flake.nix) — this only sets values for options that
+  # module declares, it doesn't import it, so every nixosSystem using
+  # common.nix needs that line too (mirrors terra's original setup).
+  home-manager.useGlobalPkgs = true;
+  home-manager.useUserPackages = true;
+  # Protects activation if a plain (non-symlink) ~/.zshrc etc. already
+  # exists from before home-manager managed it — e.g. a host where the
+  # zsh-newuser-install wizard's option (0) was used to silence itself.
+  home-manager.backupFileExtension = "hm-bak";
+  home-manager.users.darman.imports = [ ./home/common.nix ];
+
   # ---- zsh / oh-my-zsh / powerlevel10k ----
   programs.zsh = {
     enable = true;