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/templates/hosts/list.html

@@ -5,6 +5,9 @@
     <h1 class="page-title" style="margin-bottom:0;">Hosts</h1>
     <div style="display:flex;gap:0.5rem;">
         <a class="btn btn-ghost" href="/hosts/uptime">SLA</a>
+        {% if session.user_role == 'admin' %}
+        <a class="btn btn-ghost" href="/plugins/host_agent/fleet/">Agent fleet</a>
+        {% endif %}
         <a class="btn" href="/hosts/new">Add Host</a>
     </div>
 </div>
@@ -22,6 +25,7 @@
                 <th style="text-align:center;" title="Uptime last 24 hours">24h</th>
                 <th style="text-align:center;" title="Uptime last 7 days">7d</th>
                 <th style="text-align:center;" title="Uptime last 30 days">30d</th>
+                <th style="text-align:center;" title="Agent CPU / memory (latest)">Agent</th>
                 <th></th>
             </tr>
         </thead>
@@ -89,6 +93,16 @@
                     {% endif %}
                 </td>
                 {% endfor %}
+                {% set am = agent_metrics.get(host.name) %}
+                <td style="text-align:center;font-size:0.8rem;font-family:ui-monospace,monospace;">
+                    {% if am %}
+                        <span style="color:var(--text-muted);" title="CPU / memory">
+                            {{ '%.0f'|format(am.get('cpu_pct', 0)) }}% / {{ '%.0f'|format(am.get('mem_used_pct', 0)) }}%
+                        </span>
+                    {% else %}
+                        <span style="color:var(--text-dim);">—</span>
+                    {% endif %}
+                </td>
                 <td class="td-actions">
                     <a href="/hosts/{{ host.id }}/edit" class="btn btn-sm btn-ghost" style="margin-right:0.5rem;">Edit</a>
                     <form method="post" action="/hosts/{{ host.id }}/delete" style="display:inline;">