Add gt mail group subcommands:
- gt mail group list - list all groups
- gt mail group show <name> - show group details
- gt mail group create <name> [members...] - create new group
- gt mail group add <name> <member> - add member
- gt mail group remove <name> <member> - remove member
- gt mail group delete <name> - delete group
Includes validation for group names and member patterns.
Supports direct addresses, wildcards, @-patterns, and nested groups.
Part of gt-xfqh1e.7 (group commands task).
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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