feat(hosts): Phase 2+3 — agent column, fold fleet + management into Hosts

- Hosts list: new Agent column (latest CPU/mem read from the generic
  PluginMetric table — no host_agent import) + an admin "Agent fleet" button.
- /plugins/host_agent/ (old fleet page) now redirects to /hosts/ (folded into
  the hub; kept as a redirect so widgets/links don't 404).
- Agent management moved off the "settings" URL: the management page is now
  /plugins/host_agent/fleet/ ("Agent fleet" — bulk provision/install/update +
  registrations + curl install), reachable from the Hosts list. Old
  /plugins/host_agent/settings/ redirects there. Per-host management lives on
  the host detail page; this page is now explicitly the bulk/fleet view.

Milestone 70 phases 2-3. Phase 4 (plugins capability/integration split + nav
cleanup) next.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/hosts/routes.py

@@ -68,6 +68,42 @@ async def _compute_uptime(db) -> dict[str, dict]:
     return stats
 
 
+async def _agent_metrics_by_host(db, host_names: list[str]) -> dict[str, dict]:
+    """Latest agent cpu/mem per host name from the generic PluginMetric table.
+
+    Read directly (PluginMetric is a core model) so the Hosts list can show an
+    at-a-glance agent column without importing the host_agent plugin.
+    """
+    from steward.models.metrics import PluginMetric
+    if not host_names:
+        return {}
+    wanted = ("cpu_pct", "mem_used_pct")
+    subq = (
+        select(
+            PluginMetric.resource_name, PluginMetric.metric_name,
+            func.max(PluginMetric.recorded_at).label("m"),
+        )
+        .where(
+            PluginMetric.source_module == "host_agent",
+            PluginMetric.metric_name.in_(wanted),
+            PluginMetric.resource_name.in_(host_names),
+        )
+        .group_by(PluginMetric.resource_name, PluginMetric.metric_name)
+    ).subquery()
+    rows = (await db.execute(
+        select(PluginMetric).join(
+            subq,
+            (PluginMetric.resource_name == subq.c.resource_name)
+            & (PluginMetric.metric_name == subq.c.metric_name)
+            & (PluginMetric.recorded_at == subq.c.m),
+        )
+    )).scalars().all()
+    out: dict[str, dict] = {}
+    for r in rows:
+        out.setdefault(r.resource_name, {})[r.metric_name] = r.value
+    return out
+
+
 @hosts_bp.get("/")
 @require_role(UserRole.viewer)
 async def list_hosts():
@@ -105,6 +141,7 @@ async def list_hosts():
         )
         latest_dns = {r.host_id: r for r in dr.scalars()}
         uptime = await _compute_uptime(db)
+        agent_metrics = await _agent_metrics_by_host(db, [h.name for h in hosts])
 
     return await render_template(
         "hosts/list.html",
@@ -112,6 +149,7 @@ async def list_hosts():
         latest_pings=latest_pings,
         latest_dns=latest_dns,
         uptime=uptime,
+        agent_metrics=agent_metrics,
     )