fix(system_setup): update include_tasks references from .yaml to .yml

The task files were previously renamed to .yml (commits b2379e5, ce7ec1b)
but main.yml's include_tasks directives still referenced the old .yaml
names, causing role execution to fail with 'Could not find or access'
errors for extra-packages.yaml, user.yaml, and ssh.yaml.

roles/system_setup/tasks/main.yml

@@ -5,10 +5,10 @@
     name: system_maintenance
 
 # Install extra packages
-- include_tasks: extra-packages.yaml
+- include_tasks: extra-packages.yml
 
 # Create a user admin account
-- include_tasks: user.yaml
+- include_tasks: user.yml
 
 # Harden SSH configuration
-- include_tasks: ssh.yaml
+- include_tasks: ssh.yml