fix(welcome): drop Step 4's dead starter home.nix generation

`nomarchy-welcome` wrote a "starter" `~/.config/home-manager/home.nix`
for users without one. Two problems:

1. Wrong path. The installer-generated canonical home.nix lives at
   `/etc/nixos/home.nix` and is imported via the flake (both
   home-manager.users and the standalone homeConfigurations). Nothing
   in the installer flow ever reads `~/.config/home-manager/home.nix`
   — it's a dead file.
2. Broken content. The starter is missing `home.username`,
   `home.homeDirectory`, `home.stateVersion`, and doesn't import
   `nomarchy.nixosModules.home`. Even on a hand-migration path it
   wouldn't evaluate as a standalone HM config.

So in the installer flow it's dead, and in the migration flow it's
broken. Removed Step 4 entirely. The git-init step (was Step 5) is
now Step 4. Hand-migrated users follow `docs/MIGRATION.md`, which has
the correct home.nix template.

Found during Pillar 8 audit of first-boot UX.

features/scripts/utils/nomarchy-welcome

@@ -59,39 +59,9 @@ if [[ "$USER" == "nixos" ]]; then
     exit 0
 fi
 
-# 4. Starter home.nix
+# 4. Setup Local Repo (Crucial for nomarchy-env-update to work)
 echo ""
-echo "Step 4: Starter home.nix"
-HOME_NIX="$HOME/.config/home-manager/home.nix"
-if [ ! -f "$HOME_NIX" ]; then
-    echo "It looks like you don't have a ~/.config/home-manager/home.nix file yet."
-    echo "Nomarchy uses this file to manage your user-level packages and settings."
-    if gum confirm "Would you like to generate a starter home.nix?"; then
-        mkdir -p "$(dirname "$HOME_NIX")"
-        cat <<EOF > "$HOME_NIX"
-{ pkgs, ... }:
-{
-  # Nomarchy starter home.nix
-  # Add your user packages here.
-  home.packages = with pkgs; [
-    btop
-    fastfetch
-    chromium
-    # Add more packages here
-  ];
-
-  # home.stateVersion = "25.11"; # Consult docs/MIGRATION.md if you change this
-}
-EOF
-        echo "Starter home.nix generated at $HOME_NIX"
-    fi
-else
-    echo "Detected existing home.nix at $HOME_NIX. Skipping generation."
-fi
-
-# 5. Setup Local Repo (Crucial for nomarchy-env-update to work)
-echo ""
-echo "Step 5: Git Repository Check"
+echo "Step 4: Git Repository Check"
 echo "Nomarchy relies on a local git repository for declarative updates."
 if [ ! -d "/etc/nixos/.git" ]; then
     echo "Warning: /etc/nixos is not a git repository. Declarative updates might fail."
@@ -102,7 +72,7 @@ if [ ! -d "/etc/nixos/.git" ]; then
     fi
 fi
 
-# 6. Success
+# 5. Success
 echo ""
 echo "Applying all changes..."
 nomarchy-env-update