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/core/settings.py

@@ -590,13 +590,23 @@ def grade_style(grade: str | None) -> str:
     return _STATUS_STYLE.get(grade or "", "")
 
 
+# Plugins Steward can't run without, so they load whatever their stored
+# `enabled` says and Settings → Plugins offers no switch for them. The host
+# agent is one (operator, 2026-10-15): maintenance, the Hosts views, drive,
+# update and mount alerts and Docker collection all rely on it.
+ALWAYS_ON_PLUGINS = frozenset({"host_agent"})
+
+
 def to_plugins_cfg(settings: dict[str, Any]) -> dict:
-    """Assemble {plugin_name: {...config}} from all plugin.* keys."""
+    """Assemble {plugin_name: {...config}} from all plugin.* keys. An
+    always-on plugin is enabled whatever its row says, or with no row."""
     result = {}
     for key, value in settings.items():
         if key.startswith("plugin."):
             name = key[len("plugin."):]
             result[name] = value
+    for name in ALWAYS_ON_PLUGINS:
+        result[name] = {**(result.get(name) or {}), "enabled": True}
     return result