feat: nagios operator for direct exit-code severity mapping

Add ComparisonOperator.NAGIOS ("nagios") that maps Nagios exit codes
directly to alert levels (0=OK 1=WARNING 2=CRITICAL 3=UNKNOWN) without
requiring numeric warning/critical thresholds. Hysteresis is bypassed for
discrete codes. Display template defaults to "{check_name}: {output}".
_format_display() handles None threshold_value gracefully.

Add nagios_runner.status_code as a built-in default threshold config so
nagios checks alert out of the box.

Also: fix alerts.html scrolling (override html,body), make hostname a link
to /plugins#<hostname>, remove overall_status/overall_status_code/plugin_count
from nagios_runner and hbc_mini, replace with computed worst-status in
plugins.html via nagiosWorstStatus() helper.

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

hbd/server/config.py

@@ -95,6 +95,12 @@ THRESHOLD_DEFAULTS = {
                 'warning': 200,
                 'critical': 250.0,
                 'count': 3  # Optional: number of consecutive breaches before alerting
+            },
+            'nagios_runner': {
+                'status_code': {
+                    'display': '{check_name} {output}',
+                    'operator': "nagios"
+                }   
             }
         }
     }