nixos/amazon-image: Enable Amazon SSM Agent by default

Amazon-provided EC2 images do this.

See: https://docs.aws.amazon.com/systems-manager/latest/userguide/ami-preinstalled-agent.html

nixos/doc/manual/release-notes/rl-2405.section.md

@@ -333,6 +333,8 @@ The pre-existing [services.ankisyncd](#opt-services.ankisyncd.enable) has been m
 
 - [Lilypond](https://lilypond.org/index.html) and [Denemo](https://www.denemo.org) are now compiled with Guile 3.0.
 
+- The EC2 image module now enables the [Amazon SSM Agent](https://docs.aws.amazon.com/systems-manager/latest/userguide/ssm-agent.html) by default.
+
 - The following options of the Nextcloud module were moved into [`services.nextcloud.settings`](#opt-services.nextcloud.settings) and renamed to match the name from Nextcloud's `config.php`:
   - `logLevel` -> [`loglevel`](#opt-services.nextcloud.settings.loglevel),
   - `logType` -> [`log_type`](#opt-services.nextcloud.settings.log_type),

nixos/modules/virtualisation/amazon-image.nix

@@ -79,6 +79,10 @@ in
       serviceConfig.StandardOutput = "journal+console";
     };
 
+    # Amazon-issued AMIs include the SSM Agent by default, so we do the same.
+    # https://docs.aws.amazon.com/systems-manager/latest/userguide/ami-preinstalled-agent.html
+    services.amazon-ssm-agent.enable = true;
+
     # Allow root logins only using the SSH key that the user specified
     # at instance creation time.
     services.openssh.enable = true;