v3.1.0

Features

• New output.FormatYAML constant; ParseFormat recognizes yaml and yml
• New output.Format.IsStructured() helper (true for JSON/YAML) to keep
  future format additions to a single call site per command instead of N
• New base.PrintYAML / base.PrintFormatted helpers in internal/cmd/base
• All 30 commands that previously branched on outputFmt == output.FormatJSON
  now branch on outputFmt.IsStructured() and call base.PrintFormatted
• Three commands with inline json.MarshalIndent (records/update.go,
  groups/members.go, groups/zones.go) normalized to use the same
  base.PrintFormatted helper as everything else
• Added yaml:"..." struct tags (mirroring the existing json:"..." tags)
  to all CLI output schema types (internal/schema/{zone,record,user,group}.go)
  so YAML keys stay snake_case (zone_id) instead of falling back to
  gopkg.in/yaml.v3's default lowercase-no-separator naming (zoneid)
• README: documented -o yaml in both the global flags table and the
  Output Formats section
• Tests: ParseFormat and new IsStructured() coverage for yaml/yml