fix(sddm): default autoLogin off, not on with hardcoded "nomarchy"

`themes/engine/sddm.nix` defaulted `services.displayManager.autoLogin`
to `enable = true; user = "nomarchy";` (both mkDefault). The installer
flow overrode both with the real username at normal priority, so this
was invisible there — but a hand-migrated user (per docs/MIGRATION.md)
who imported `nomarchy.nixosModules.system` without setting
`autoLogin.user` would auto-login as a nonexistent "nomarchy" user and
SDDM would error. `docs/MIGRATION.md` even documented the override as a
post-import chore.

Flipped the default to `enable = lib.mkDefault false`. Installer
generates `enable = true` directly so its flow is unchanged. Migration
flow now gets the safe default — opt-in instead of opt-out — and the
docs row is updated to reflect the new shape.

The hardcoded "nomarchy" username fallback for `autoLogin.user` is the
same class of bug as the impermanence persistence block was. A future
roadmap row to consolidate "primary user" across impermanence,
autoLogin, and any future modules might be worthwhile, but it's
deferred — this commit is the immediate fix.

Found during Pillar 8 audit of first-boot UX.

docs/MIGRATION.md

@@ -210,7 +210,7 @@ and it's whichever has higher Nix priority. Fix these explicitly:
 | Graphics | `hardware.graphics.enable = true` (was `hardware.opengl`) | Probably already enabled — fine |
 | User groups | needs `video render networkmanager` | Add to your `users.users.<user>.extraGroups` |
 | `/etc/os-release` | `ID=nomarchy`, `NAME=Nomarchy` | A few third-party scripts grep `ID=nixos` — adjust them or rely on `ID_LIKE` (TBD) |
-| autoLogin | `enable = true; user = "nomarchy";` (mkDefault) | Override with `services.displayManager.autoLogin.user = "<your user>"` or disable |
+| autoLogin | `enable = false; user = "nomarchy";` (mkDefault) | Off by default — opt in with `services.displayManager.autoLogin = { enable = true; user = "<your user>"; };` if you want it |
 
 Impermanence is **off** unless you set `nomarchy.system.impermanence.enable = true`,
 and it requires a BTRFS layout with a `root-blank` snapshot. Don't enable it

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

themes/engine/sddm.nix

@@ -27,8 +27,12 @@ in
 
   services.displayManager.defaultSession = lib.mkDefault "hyprland-uwsm";
 
+  # autoLogin defaults off so hand-migrated configs (no installer-written
+  # username) don't try to log in as a nonexistent "nomarchy" user. The
+  # installer-generated system.nix sets both `enable = true;` and
+  # `user = "$USERNAME";` at normal priority, overriding these defaults.
   services.displayManager.autoLogin = {
-    enable = lib.mkDefault true;
+    enable = lib.mkDefault false;
     user = lib.mkDefault "nomarchy";
   };