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>

README.md

@@ -299,6 +299,8 @@ Available variables:
 | `{op_symbol}` | Comparison operator (`>`, `<`, `>=`, …) |
 | `{check_name}` | Prefix stripped by generic matching (see below) |
 | `{metric_name}` | Full field name within the plugin data |
+| `{output}` | For `nagios_runner` generic matches: the matched check's status text (alias for `{check_name}_output`) |
+| `{status}` | For `nagios_runner` generic matches: the matched check's status name — OK/WARNING/CRITICAL/UNKNOWN (alias for `{check_name}_status`) |
 | any plugin field | Any other field present in the plugin's data |
 
 ### Generic Threshold Matching