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/plugins.html

@@ -29,7 +29,7 @@
                      background:color-mix(in srgb,var(--red) 15%,var(--bg));color:var(--red);">Error</span>
       {% elif plugin._enabled %}
         <span style="font-size:0.72rem;padding:0.1em 0.45em;border-radius:3px;
-                     background:color-mix(in srgb,var(--green) 12%,var(--bg));color:var(--green);">Enabled</span>
+                     background:color-mix(in srgb,var(--green) 12%,var(--bg));color:var(--green);">{{ "Built in" if plugin._always_on else "Enabled" }}</span>
       {% else %}
         <span style="font-size:0.72rem;padding:0.1em 0.45em;border-radius:3px;
                      background:var(--bg-elevated);color:var(--text-muted);">Disabled</span>