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/timezone.yml

@@ -0,0 +1,24 @@
+---
+# ============================================================================
+# Timezone and locale
+# ============================================================================
+
+- name: Set system timezone
+  community.general.timezone:
+    name: "{{ baseline_timezone }}"
+
+- name: Ensure locale package present (Debian)
+  ansible.builtin.apt:
+    name: locales
+    state: present
+  when: ansible_os_family == "Debian"
+
+- name: Generate locale
+  community.general.locale_gen:
+    name: "{{ baseline_locale }}"
+    state: present
+
+- name: Set system locale
+  ansible.builtin.command:
+    cmd: "localectl set-locale LANG={{ baseline_locale }}"
+  changed_when: false