fix(apm): set default environment filter to 'prod' for services list

Changed the default value for --env flag from empty string to "prod"
for the apm services list command to improve usability.

- cmd/apm.go: Updated apmServicesListCmd --env default from "" to "prod"

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

Author: platinummonkey

cmd/apm.go

@@ -428,7 +428,7 @@ var (
 
 func init() {
 	// Services list flags
-	apmServicesListCmd.Flags().StringVar(&envFilter, "env", "", "Environment filter")
+	apmServicesListCmd.Flags().StringVar(&envFilter, "env", "prod", "Environment filter")
 	apmServicesListCmd.Flags().Int64Var(&startTime, "start", time.Now().Add(-1*time.Hour).Unix(), "Start time (Unix timestamp)")
 	apmServicesListCmd.Flags().Int64Var(&endTime, "end", time.Now().Unix(), "End time (Unix timestamp)")