feat(dashboard): unified Hosts widget + grouped widget picker

Bring the dashboard in line with the unified host IA + improved displays.

- New core "Hosts — Overview" widget (/hosts/overview/widget): one row per host
  combining monitor status (ping dot + latency, uptime 24h) with the agent
  glance (CPU / memory / disk root + stale flag), each row linking to the host
  hub. Reads agent data from the generic PluginMetric table via a core-safe
  _agent_overview_by_host helper (no host_agent import); freshness vs the
  plugin's stale window. The granular Ping/DNS/Uptime/Agent widgets stay.
- Group the add-widget picker into Core monitors / Monitoring capabilities /
  Integrations (a `group` field on every WIDGET_REGISTRY entry + section
  headings in _edit_panels.html), matching the Settings → Plugins taxonomy.
- Fix the agent fleet widget rows to link to the host hub (/hosts/<id>) instead
  of the old /plugins/host_agent/<id>/ page.

Scribe #903.

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

steward/templates/dashboard/_edit_panels.html

@@ -37,8 +37,13 @@
   <div>
     <div class="section-title" style="margin-bottom:0.75rem;">Available Widgets</div>
     {% if addable %}
+    {% set _groups = [("core", "Core monitors"), ("capability", "Monitoring capabilities"), ("integration", "Integrations")] %}
+    {% for _gkey, _glabel in _groups %}
+    {% set _gw = addable | selectattr("group", "equalto", _gkey) | list %}
+    {% if _gw %}
+    <div style="font-size:0.72rem;text-transform:uppercase;letter-spacing:0.04em;color:var(--text-dim);margin:0.85rem 0 0.35rem;">{{ _glabel }}</div>
     <div style="display:grid;gap:0.5rem;">
-      {% for w in addable %}
+      {% for w in _gw %}
       <div class="card" style="margin-bottom:0;padding:0;overflow:hidden;">
         <details>
           <summary style="display:flex;align-items:center;justify-content:space-between;gap:0.75rem;
@@ -97,6 +102,8 @@
       </div>
       {% endfor %}
     </div>
+    {% endif %}
+    {% endfor %}
     {% else %}
     <div style="color:var(--text-muted);font-size:0.85rem;padding:1rem 0;">
       No plugin widgets available. Enable plugins in Settings → Plugins.