feat: nagios_runner improvements and alerts page fixes

- nagios_runner: remove overall_status/overall_status_code/plugin_count fields;
  each command still reports its own <name>_status and <name>_status_code
- threshold: expose {output} and {status} aliases in display templates for
  nagios_runner generic matches (mapped from <check_name>_output/status)
- alerts.html: fix scrolling by overriding html,body height/overflow (style.css
  sets both); make hostname a link to /plugins/<hostname>

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hbd/server/templates/alerts.html

@@ -4,7 +4,7 @@
 
   <style>
 
-    body {
+    html, body {
       height: auto;
       overflow-y: auto;
     }
@@ -175,8 +175,12 @@
 
     .alert-hostname {
       font-weight: bold;
-      color: #333;
+      color: #0066cc;
       font-size: 1.1em;
+      text-decoration: none;
+    }
+    .alert-hostname:hover {
+      text-decoration: underline;
     }
 
     .alert-metric {
@@ -433,7 +437,7 @@
             <div class="alert-main">
               <div class="alert-header">
                 <span class="alert-level ${level}">${alert.level}</span>
-                <span class="alert-hostname">${alert.hostname}</span>
+                <a class="alert-hostname" href="/plugins/${alert.hostname}">${alert.hostname}</a>
               </div>
               <div class="alert-metric">${alert.metric_path}</div>
               <div class="alert-details">