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/handlers/main.yml

@@ -0,0 +1,20 @@
+---
+- name: Restart chrony
+  ansible.builtin.systemd:
+    name: "{{ baseline_chrony_service }}"
+    state: restarted
+
+- name: Restart sshd
+  ansible.builtin.systemd:
+    name: "{{ baseline_sshd_service }}"
+    state: restarted
+
+- name: Reload sysctl
+  ansible.builtin.command:
+    cmd: sysctl --system
+  changed_when: false
+
+- name: Restart systemd-journald
+  ansible.builtin.systemd:
+    name: systemd-journald
+    state: restarted