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>

plugins/host_agent/templates/settings_list.html

@@ -1,9 +1,16 @@
 {% extends "base.html" %}
 {% from "_macros.html" import crumbs %}
-{% block title %}Host Agent — Steward{% endblock %}
-{% block breadcrumb %}{{ crumbs([("Host Agents", "/plugins/host_agent/"), ("Settings", "")]) }}{% endblock %}
+{% block title %}Agent fleet — Steward{% endblock %}
+{% block breadcrumb %}{{ crumbs([("Hosts", "/hosts/"), ("Agent fleet", "")]) }}{% endblock %}
 {% block content %}
-<h1 class="page-title">Host Agent — Registered Hosts</h1>
+<div style="display:flex;align-items:center;justify-content:space-between;margin-bottom:1rem;gap:1rem;flex-wrap:wrap;">
+  <h1 class="page-title" style="margin-bottom:0;">Agent fleet</h1>
+  <a href="/hosts/" class="btn btn-ghost btn-sm">← Hosts</a>
+</div>
+<p style="color:var(--text-muted);font-size:0.84rem;margin:-0.4rem 0 1.25rem;">
+  Bulk agent operations across your inventory. For a single host, use its
+  <a href="/hosts/">host page</a> — provisioning and metrics live there.
+</p>
 
 {% if new_token %}
 <div class="card" style="background:var(--accent-bg);">
@@ -58,7 +65,7 @@
       No managed SSH key yet — Steward needs one to log into hosts as <code>steward</code>.
     </span>
     <form method="post" action="/settings/ansible/generate-key" style="margin:0;">
-      <input type="hidden" name="next" value="/plugins/host_agent/settings/">
+      <input type="hidden" name="next" value="/plugins/host_agent/fleet/">
       <button type="submit" class="btn btn-sm">Generate managed key</button>
     </form>
   </div>