fix: support plugin-level enabled: false in threshold config

Setting enabled: false at the plugin level (e.g. memory_monitor: {enabled: false})
was silently ignored because the non-dict value was skipped by the metric parser,
leaving THRESHOLD_DEFAULTS entries active.

- _parse_plugin_thresholds: detect plugin-level enabled/enable flag and delete
  all matching entries from target_dict (covers legacy and default config paths)
- _parse_multi_config named configs: inject disabled stubs from effective_defaults
  into raw_overrides so the merge step overwrites inherited defaults
- Accept 'enable' as a tolerated alias for 'enabled' in both code paths

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

hbd/server/threshold.py

@@ -492,7 +492,27 @@ class ThresholdChecker:
             raw_overrides: Dict[str, ThresholdConfig] = {}
             thresholds_config = config_data["thresholds"]
             for plugin_name, plugin_thresholds in thresholds_config.items():
-                if isinstance(plugin_thresholds, dict):
+                if not isinstance(plugin_thresholds, dict):
+                    continue
+                plugin_enabled = plugin_thresholds.get('enabled', plugin_thresholds.get('enable', True))
+                if not plugin_enabled:
+                    # raw_overrides is empty at this point so there's nothing to delete.
+                    # Instead, inject disabled stubs for every matching effective_default so
+                    # the merge step overwrites the inherited defaults.
+                    for key, tc in effective_defaults.items():
+                        if key.startswith(f"{plugin_name}."):
+                            raw_overrides[key] = ThresholdConfig(
+                                metric_path=key,
+                                warning=tc.warning,
+                                critical=tc.critical,
+                                operator=tc.operator.value,
+                                enabled=False,
+                            )
+                    logger.info(
+                        "Plugin-level disable in config '%s': disabled all thresholds for %s",
+                        config_name, plugin_name,
+                    )
+                else:
                     self._parse_plugin_thresholds(plugin_name, plugin_thresholds, target_dict=raw_overrides)
             self.threshold_raw_configs[config_name] = raw_overrides
 
@@ -571,6 +591,15 @@ class ThresholdChecker:
             self._parse_rtt_thresholds(thresholds, target_dict)
             return
 
+        # Plugin-level enabled: false (also accept 'enable' as a common typo) removes all
+        # thresholds for this plugin — e.g. memory_monitor: {enabled: false}.
+        plugin_enabled = thresholds.get('enabled', thresholds.get('enable', True))
+        if not plugin_enabled:
+            for key in [k for k in target_dict if k.startswith(f"{plugin_name}.")]:
+                del target_dict[key]
+            logger.info("Plugin-level disable: removed all thresholds for %s", plugin_name)
+            return
+
         for metric_name, threshold_config in thresholds.items():
             if not isinstance(threshold_config, dict):
                 continue