feat: composable threshold_config list for per-host threshold layering

threshold_config in the hosts section now accepts a list of named
configs applied left-to-right on top of the defaults, so focused
override profiles can be mixed without duplication. Single-string
and legacy host_threshold_mapping forms are unchanged.

- Add threshold_raw_configs to store per-config overrides separately
- Normalise threshold_config to list on parse (string or list)
- get_thresholds_for_host folds the list over the default base
- Update README and docs/THRESHOLD_ALERTING.md with examples

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -267,6 +267,41 @@ All plugin metrics can be thresholded:
 - **Network**: errors_total, dropped packets, connection counts
 - **Nagios**: exit_code mapping (0=OK, 1=WARNING, 2=CRITICAL)
 
+### Per-Host Threshold Profiles
+
+Named threshold configurations let different hosts use different limits. A host's `threshold_config` can be a single name or a **list** — lists are applied left-to-right so profiles compose without duplication:
+
+```yaml
+threshold_configs:
+  default:
+    thresholds:
+      cpu_monitor:
+        cpu_percent: {warning: 80, critical: 90}
+      memory_monitor:
+        memory_percent: {warning: 85, critical: 95}
+
+  tight_cpu:           # override CPU limits only
+    thresholds:
+      cpu_monitor:
+        cpu_percent: {warning: 60, critical: 75}
+
+  db_disk:             # add a database partition check
+    thresholds:
+      disk_monitor:
+        partitions:
+          /var/lib/postgresql:
+            percent: {warning: 75, critical: 88}
+
+hosts:
+  web-01:
+    threshold_config: default          # single profile
+
+  db-01:
+    threshold_config: [tight_cpu, db_disk]   # layered: CPU override + extra disk check
+```
+
+Each named config's overrides are applied in order on top of the defaults. Metrics not mentioned in a profile are inherited unchanged.
+
 See [docs/THRESHOLD_ALERTING.md](docs/THRESHOLD_ALERTING.md) for comprehensive documentation including best practices, troubleshooting, and advanced configuration.
 
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