feat: add production-ready Prometheus & Alertmanager configs

- prometheus.yml: Service discovery, alerting, multi-job scraping
- alertmanager.yml: Routing tree, inhibition rules, multi-channel
- node-exporter.yml: 30+ alert rules (CPU, memory, disk, network, system)
- File-based service discovery for dynamic host management
- Updated README with usage docs and alert catalog

Alert categories: availability, resource saturation, disk predictive,
I/O latency, network errors, clock sync, OOM detection, conntrack

prometheus/targets/node-exporter.yml

@@ -0,0 +1,30 @@
+# Node Exporter Targets
+# File-based service discovery for node-exporter
+# Prometheus watches this file and reloads targets automatically
+#
+# Add your hosts below:
+# - Each target should be hostname:port (default port 9100)
+# - Labels are applied to all metrics from that target
+# - Changes are picked up automatically (no Prometheus restart needed)
+
+# Example configuration:
+# - targets:
+#     - 'web-server-01:9100'
+#     - 'web-server-02:9100'
+#   labels:
+#     role: 'web'
+#     datacenter: 'us-east-1'
+#
+# - targets:
+#     - 'db-primary:9100'
+#     - 'db-replica-01:9100'
+#   labels:
+#     role: 'database'
+#     datacenter: 'us-east-1'
+
+# Your targets:
+- targets:
+    - 'localhost:9100'
+  labels:
+    role: 'prometheus'
+    env: 'production'