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.

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";
   };