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

system/motd.nix

@@ -0,0 +1,28 @@
+{ config, lib, self, ... }:
+
+let
+  hostname = config.networking.hostName;
+  domain = config.networking.domain or "";
+  fqdn = if domain != "" then "${hostname}.${domain}" else hostname;
+
+  # Get commit hash (handles both clean and dirty trees)
+  shortRev = self.shortRev or self.dirtyShortRev or "unknown";
+
+  # Format timestamp from lastModified (Unix timestamp)
+  # lastModifiedDate is in format "YYYYMMDDHHMMSS"
+  dateStr = self.sourceInfo.lastModifiedDate or "unknown";
+  formattedDate = if dateStr != "unknown" then
+    "${builtins.substring 0 4 dateStr}-${builtins.substring 4 2 dateStr}-${builtins.substring 6 2 dateStr} ${builtins.substring 8 2 dateStr}:${builtins.substring 10 2 dateStr}"
+  else
+    "unknown";
+
+  banner = ''
+    ####################################
+     ${fqdn}
+     Commit: ${shortRev} (${formattedDate})
+    ####################################
+  '';
+in
+{
+  users.motd = lib.mkDefault banner;
+}