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>

tests/plugins/host_agent/test_mcp_tools.py

@@ -18,5 +18,28 @@ def test_agent_state():
 def test_plugin_exposes_the_mcp_hooks():
     reg = MagicMock()
     agent_plugin.register_mcp_tools(reg)
-    assert [c.args[0].__name__ for c in reg.call_args_list] == ["list_agents"]
+    assert [c.args[0].__name__ for c in reg.call_args_list] == [
+        "list_agents", "list_maintenance_plans", "get_maintenance_job", "run_maintenance"]
+    roles = {c.args[0].__name__: c.kwargs["role"] for c in reg.call_args_list}
+    assert roles["run_maintenance"].value == "admin"           # as Update now on the page
+    assert roles["get_maintenance_job"].value == "viewer"
     assert callable(agent_plugin.mcp_overview)
+
+
+def test_a_job_row_says_what_it_did_and_when_it_gives_up():
+    from plugins.host_agent.mcp_tools import _job_row
+    job = NS(id="j1", host_id="h1", state="rebooting", state_at=NOW - timedelta(minutes=5),
+             reboot_policy="if_required", packages=12, reboot_needed=True, back_at=None,
+             node_id="n1", drain_host_id="h2", detail=None, created_by=None, run_id="r1",
+             created_at=NOW - timedelta(minutes=20), finished_at=None)
+    row = _job_row(job, {"h1": "web-1", "h2": "mgr-1"}, NOW)
+    assert (row["host"], row["drained_by"], row["label"], row["active"]) == (
+        "web-1", "mgr-1", "Rebooting", True)
+    assert row["started_by"] == "maintenance plan" and row["state_age_s"] == 300
+    assert row["gives_up_at"] == (NOW + timedelta(minutes=25)).isoformat().replace("+00:00", "Z")
+
+    done = NS(**{**job.__dict__, "state": "done", "back_at": NOW, "finished_at": NOW,
+                 "created_by": "u1", "run_id": None})
+    row = _job_row(done, {"h1": "web-1", "u1": "alice"}, NOW)
+    assert row["summary"] == "Done: 12 packages updated, rebooted"
+    assert row["started_by"] == "alice" and row["gives_up_at"] is None and not row["active"]