Migrate all CLI command handlers from cobra's Run to RunE, replacing
fmt.Println(err) + os.Exit(1) patterns with idiomatic error returns.
This improves testability and lets cobra handle errors consistently
via the existing SilenceUsage/SilenceErrors configuration.
Also refactor getDuration() and getTime() helpers to return errors
instead of calling os.Exit(1).
Fix a bug in group.go where the error from inspector.Groups() was
never checked, causing silent failures (e.g. on Redis connection
errors) to be misreported as "No groups found".
