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>

docs/THRESHOLD_ALERTING.md

@@ -1110,33 +1110,6 @@ hosts:
   db-02:
     threshold_config: [tight_memory, db_disk]
 ```
-
-### Backward Compatibility
-
-The legacy single threshold configuration is fully supported:
-
-```yaml
-# Old format - still works
-thresholds:
-  cpu_monitor:
-    cpu_percent:
-      warning: 80.0
-      critical: 90.0
-```
-
-This is equivalent to:
-
-```yaml
-# New format
-threshold_configs:
-  default:
-    thresholds:
-      cpu_monitor:
-        cpu_percent:
-          warning: 80.0
-          critical: 90.0
-```
-
 ### Configuration Priority
 
 1. **Host `threshold_config` (list)**: Layer each named config's overrides left-to-right on top of the defaults