feat(branding): tuigreet greeting + branded MOTD

Round 2 of distro branding, text half: tuigreet now shows
'Welcome to <distroName>', and users.motd carries a branded banner that
doubles as a cheat sheet for the distro helpers (sys-update, home-update,
nomarchy-theme-sync, SUPER+?). Both key off system.nixos.distroName so the
brand name stays in one place.

Deferred (need an asset / a boot test): ISO splashImage/grubTheme art and
the distroId switch.

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

README.md

@@ -316,8 +316,10 @@ close · `SUPER+1..9` workspaces · `Print` region screenshot.
   `nomarchy.osd.*`, media keys drive `swayosd-client`, themed from the JSON)
 - **Distro branding, round 2:** `distroName = "Nomarchy"` ships
   (os-release `PRETTY_NAME`, systemd-boot entries, ISO menu label).
+  ✓ tuigreet greeting (`Welcome to <distroName>`) and a branded `users.motd`
+  (doubling as a helper cheat sheet), both keyed off `distroName`.
   Remaining: `isoImage.splashImage`/`grubTheme` art for the ISO boot
-  screen, tuigreet/MOTD text, and the `distroId` question (it changes
+  screen (needs a designed asset), and the `distroId` question (it changes
   `DEFAULT_HOSTNAME` and upstream `isNixos` checks — needs a test pass;
   nixos-* CLI names stay regardless)
 - **Theme-switch feedback:** ✓ the "rebuilding…" notification is now