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>

CLAUDE.md

@@ -35,6 +35,21 @@ nix build .#create-host
 
 Do not automatically deploy changes. Deployments are usually done by updating the master branch, and then triggering the auto update on the specific host.
 
+### Testing Feature Branches on Hosts
+
+All hosts have the `nixos-rebuild-test` helper script for testing feature branches before merging:
+
+```bash
+# On the target host, test a feature branch
+nixos-rebuild-test boot <branch-name>
+nixos-rebuild-test switch <branch-name>
+
+# Additional arguments are passed through to nixos-rebuild
+nixos-rebuild-test boot my-feature --show-trace
+```
+
+When working on a feature branch that requires testing on a live host, suggest using this command instead of the full flake URL syntax.
+
 ### Flake Management
 
 ```bash

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;
+}