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/playbooks/day0_linux_baseline.yml

@@ -0,0 +1,23 @@
+---
+# ============================================================================
+# day0_linux_baseline.yml
+# ----------------------------------------------------------------------------
+# Applies the mk-labs Linux baseline (linux-baseline role) to one or more
+# hosts. Idempotent and safe to re-run.
+#
+# Usage:
+#   ansible-playbook playbooks/day0_linux_baseline.yml
+#   ansible-playbook playbooks/day0_linux_baseline.yml -e target=figment
+#   ansible-playbook playbooks/day0_linux_baseline.yml -e target=semaphore_server
+#
+# To trigger an opt-in full system upgrade:
+#   ansible-playbook playbooks/day0_linux_baseline.yml \
+#     -e target=figment -e 'baseline_features={"full_upgrade": true}'
+# ============================================================================
+
+- name: Apply mk-labs Linux baseline
+  hosts: "{{ target | default('all') }}"
+  become: true
+  gather_facts: true
+  roles:
+    - linux-baseline