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/packages.yml

@@ -0,0 +1,32 @@
+---
+# ============================================================================
+# Package baseline
+# ============================================================================
+
+- name: Update apt cache (Debian family)
+  ansible.builtin.apt:
+    update_cache: true
+    cache_valid_time: 3600
+  when: ansible_os_family == "Debian"
+
+- name: Install baseline packages (common)
+  ansible.builtin.package:
+    name: "{{ baseline_packages_common }}"
+    state: present
+
+- name: Install baseline packages (OS family)
+  ansible.builtin.package:
+    name: "{{ baseline_packages_family }}"
+    state: present
+
+- name: Install baseline packages (host extras)
+  ansible.builtin.package:
+    name: "{{ baseline_packages_extra }}"
+    state: present
+  when: baseline_packages_extra | length > 0
+
+- name: Perform full system upgrade (opt-in)
+  ansible.builtin.package:
+    name: "*"
+    state: latest  # noqa package-latest - intentional under feature flag
+  when: baseline_features.full_upgrade | bool