feat: the host agent is always on; the MCP sees and starts host maintenance (#5834)

#5834 "The host agent is always on, and the MCP sees a host's
maintenance", step 1 of milestone 520 "The MCP sees what Steward has
grown".

The host agent is a core system (operator, 2026-10-15: "it's very core
to the functionality of Steward as a whole and shouldn't be considered
something that can be toggled off"). ALWAYS_ON_PLUGINS in core settings
makes to_plugins_cfg report it enabled whatever its stored row says, so
it always loads and its MCP tools are always listed. Settings -> Plugins
shows it as "Built in: always on" with no switch, and a save keeps it
enabled. Its code and URLs stay under plugins/host_agent: every
installed agent posts to /plugins/host_agent/ingest.

Maintenance through the MCP:
- list_maintenance_plans: each plan's group, weekly slot, reboot policy,
  next slot, and its latest run (hosts, how far it got, who it skipped
  and why, how it ended).
- get_maintenance_job(host | job): a host's jobs, newest and running
  first, or one job: state, packages updated, reboot, drained by, why it
  failed, when its current state times out.
- run_maintenance(host, reboot="if_required" | "never"): admin, as the
  host page's Update now; refuses a busy host or one with no helper;
  audited "<user> via MCP".
- The overview names a host whose latest job in the last week failed or
  timed out.

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

steward/templates/settings/plugin_detail.html

@@ -43,7 +43,16 @@
 
 <form method="post" action="/settings/plugins/{{ plugin._dir }}/" style="max-width:720px;">{{ csrf_field() }}
 
-  {# ── Enable toggle ────────────────────────────────────────────────────────── #}
+  {# ── Enable toggle; a built-in plugin has none (always on) ─────────────────── #}
+  {% if plugin._always_on %}
+  <div class="card" style="padding:1rem 1.25rem;margin-bottom:1rem;">
+    <div style="font-weight:500;">Built in: always on</div>
+    <div style="font-size:0.8rem;color:var(--text-muted);margin-top:0.4rem;">
+      Steward relies on this one (hosts, maintenance, drive and update alerts, Docker collection), so it can't
+      be turned off. Its settings below still apply.
+    </div>
+  </div>
+  {% else %}
   <div class="card" style="padding:1rem 1.25rem;margin-bottom:1rem;">
     <div style="display:flex;align-items:center;gap:0.75rem;">
       <input type="checkbox"
@@ -58,6 +67,7 @@
       Enabling loads the plugin straight away. Disabling takes effect after a restart.
     </div>
   </div>
+  {% endif %}
 
   {# ── Config fields ─────────────────────────────────────────────────────────── #}
   {% if plugin.get('config') %}