feat(ansible): add linux-baseline role and day0_linux_baseline playbook

Introduces a single, idempotent baseline role to supersede the
overlapping day0-baseline and common roles. Capabilities are
feature-flagged so they can be toggled per-host:

  - packages (common + OS-family + per-host extras)
  - timezone + locale
  - chrony time sync against sundial
  - baseline users (jarvis admin account with SSH key + NOPASSWD sudo)
  - SSH hardening via /etc/ssh/sshd_config.d/ drop-in
  - unattended security upgrades (Debian family)
  - sysctl drop-in at /etc/sysctl.d/99-mk-labs.conf
  - journald retention caps
  - branded MOTD

Ubuntu/Debian is first-class; vars/RedHat.yml provides a placeholder
for future distros via the ansible_os_family pattern.

The legacy day0-baseline and common roles remain in place for now and
will be removed during the playbook cleanup sweep, alongside the
existing playbook naming inconsistencies.

ansible/roles/linux-baseline/tasks/time_sync.yml

@@ -0,0 +1,25 @@
+---
+# ============================================================================
+# Time synchronisation via chrony
+# ============================================================================
+
+- name: Install chrony
+  ansible.builtin.package:
+    name: chrony
+    state: present
+
+- name: Deploy chrony configuration
+  ansible.builtin.template:
+    src: chrony.conf.j2
+    dest: "{{ baseline_chrony_config_path }}"
+    owner: root
+    group: root
+    mode: "0644"
+    backup: true
+  notify: Restart chrony
+
+- name: Ensure chrony is enabled and running
+  ansible.builtin.systemd:
+    name: "{{ baseline_chrony_service }}"
+    state: started
+    enabled: true