feat: include active alert count in MCP server instructions

Add InstructionsFunc callback to ServerConfig, called during each
initialize handshake to generate dynamic instructions. The lab-monitoring
server uses this to query Alertmanager and include a count of active
non-silenced alerts, so the LLM can proactively inform the user.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/lab-monitoring/main.go

@@ -168,10 +168,15 @@ func runServe(c *cli.Context) error {
 
 	logger := log.New(os.Stderr, "[mcp] ", log.LstdFlags)
 	config := mcp.DefaultMonitoringConfig()
-	server := mcp.NewGenericServer(logger, config)
 
 	prom := monitoring.NewPrometheusClient(c.String("prometheus-url"))
 	am := monitoring.NewAlertmanagerClient(c.String("alertmanager-url"))
+
+	config.InstructionsFunc = func() string {
+		return monitoring.AlertSummary(am)
+	}
+
+	server := mcp.NewGenericServer(logger, config)
 	monitoring.RegisterHandlers(server, prom, am)
 
 	transport := c.String("transport")