feat(mksystem): consumer-friendly home-manager resolution

mkSystem resolved the user's home config only from this repo's
users/<user>/, so an external consumer's user got no home-manager setup.
Add a `home` param (a path) and resolve in priority order: caller `home`
-> internal users/<user>/home-manager.nix -> OmniXY's home.nix. Internal
hosts keep their shims (unchanged); consumers now get a working home by
default. Unblocks the consumer template.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/mksystem.nix

@@ -56,6 +56,10 @@ name:
   user,
   devEnv ? false,
   modules ? [ ],
+  # Home-manager config for `user`, as a PATH. External consumers pass
+  # their own (e.g. `home = ./home.nix`). When null, fall back to this
+  # repo's users/<user>/home-manager.nix, then to Omnixient's home.nix.
+  home ? null,
   # Extra args merged into the system-level specialArgs. Used by hosts
   # whose configuration.nix expects values beyond the defaults (e.g.
   # omni's configuration.nix takes `settings` from settings.nix).
@@ -84,10 +88,21 @@ let
       throw "mksystem: no host config for '${name}' — add hosts/${name}/ or pass one via `modules` (looked at ${toString hostDir} and ${toString hostFile})";
 
   userNixosConfig = ../users/${user}/nixos.nix;
-  userHMConfig = ../users/${user}/home-manager.nix;
-
   hasUserNixosConfig = builtins.pathExists userNixosConfig;
-  hasUserHMConfig = builtins.pathExists userHMConfig;
+
+  # Home-manager config for `user`, resolved in priority order so both
+  # internal hosts and external consumers get a working home:
+  #   1. caller-supplied `home` (consumer override), else
+  #   2. this repo's users/<user>/home-manager.nix (internal hosts), else
+  #   3. Omnixient's default home.nix (consumer whose user isn't in this tree).
+  internalUserHM = ../users/${user}/home-manager.nix;
+  userHMConfig =
+    if home != null then
+      home
+    else if builtins.pathExists internalUserHM then
+      internalUserHM
+    else
+      ../home.nix;
 in
 
 nixpkgs.lib.nixosSystem {
@@ -149,9 +164,11 @@ nixpkgs.lib.nixosSystem {
     # bootstrap.
     ../modules/home-manager-bootstrap.nix
   ]
-  ++ nixpkgs.lib.optional hasUserHMConfig {
-    home-manager.users.${user} = import userHMConfig;
-  }
+  ++ [
+    # Home config for `user` — always wired now that userHMConfig always
+    # resolves (caller `home`, internal shim, or Omnixient's home.nix).
+    { home-manager.users.${user} = import userHMConfig; }
+  ]
   # dev-env is opt-in.
   ++ nixpkgs.lib.optional devEnv ../modules/dev-env
   # Caller-supplied extras.