feat(host_agent): a root helper the agent can start, for updates, reboots and drains only (#5766)

#5766 "A root helper the agent can start, for a fixed set of maintenance
actions and nothing else", milestone 514.

The agent runs unprivileged under NoNewPrivileges, so it can't update or
reboot its host. It now leaves a request in /var/lib/steward-agent/requests,
the one path its unit lets it write; steward-maint.path starts maint.py as
root, which reads it without following links, accepts only update, reboot,
drain or undrain, deletes it, and writes the outcome to
/var/lib/steward-maint/results. The agent (1.17.0) reports each new result
state once as a `maintenance` event, re-checks pending updates when an update
finishes, and says in its metadata whether the helper is installed.

The units and the helper are served from one copy at
/plugins/host_agent/files/<name> (a fixed list), and install.sh plus the
install, provision and update playbooks all fetch them, replacing three
inline copies of the unit. Update agent installs the helper on existing
hosts; the Agent panel says under Manage agent whether it is there.

Steward handing out jobs is #5767; the cadence is #5768.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/host_agent/templates/panel.html

@@ -78,6 +78,11 @@
       Link an Ansible target (below) to update the agent from here.
     </p>
     {% endif %}
+    {# The root helper that carries out scheduled updates and reboots (#5766). #}
+    <p style="font-size:0.8rem;margin:0.6rem 0 0;color:var(--text-dim);">
+      {% if reg.maint_helper %}Maintenance helper installed: Steward can update and reboot this host.
+      {% else %}No maintenance helper yet: Update agent installs it, so Steward can update and reboot this host.{% endif %}
+    </p>
 
     {% if ansible_available and target and managed_key_set %}
     <details style="margin-top:0.7rem;">