system: add MOTD banner with hostname and commit info

Displays FQDN and flake commit hash with timestamp on login.
Templates can override with their own MOTD via mkDefault.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

system/default.nix

@@ -4,6 +4,7 @@
     ./acme.nix
     ./autoupgrade.nix
     ./monitoring
+    ./motd.nix
     ./packages.nix
     ./nix.nix
     ./root-user.nix