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/NAGIOS_INTEGRATION.md

@@ -104,11 +104,6 @@ The `nagios_runner` plugin collects:
 - `{name}_{metric}_min` - Minimum value (if present)
 - `{name}_{metric}_max` - Maximum value (if present)
 
-**Overall:**
-- `overall_status` - Worst status from all commands
-- `overall_status_code` - Worst status code
-- `plugin_count` - Number of Nagios plugins executed
-
 ## Configuration Options
 
 ```yaml