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/preflight.yml

@@ -0,0 +1,18 @@
+---
+# ============================================================================
+# Preflight: fail fast on unsupported platforms.
+# ============================================================================
+
+- name: Assert a supported OS family
+  ansible.builtin.assert:
+    that:
+      - ansible_os_family in ["Debian", "RedHat"]
+    fail_msg: >-
+      linux-baseline currently supports the Debian and RedHat OS families.
+      Host {{ inventory_hostname }} is running {{ ansible_distribution }}
+      {{ ansible_distribution_version }} ({{ ansible_os_family }}).
+    success_msg: "Detected {{ ansible_distribution }} {{ ansible_distribution_version }}."
+
+- name: Gather package facts
+  ansible.builtin.package_facts:
+    manager: auto