Add Slack notify helper for deployment alerts

lib/notify.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+# Send a Slack notification via webhook.
+# Usage: notify "Deployment of v1.2.3 succeeded" "#deployments"
+
+notify() {
+    local msg="$1"
+    local channel="${2:-#ops}"
+    curl -sf -X POST "$SLACK_WEBHOOK_URL" \
+        -H "Content-Type: application/json" \
+        -d "{\"channel\": \"$channel\", \"text\": \"$msg\"}"
+}