feat: add group management commands

Implemented roadmap #5: Group management commands with full CRUD.

Features:
- group create <name> [members] - Create new group with optional initial members
- group get <uuid> - Display group details (uuid, name_hash, members, timestamps)
- group update <uuid> <members> - Replace entire member list
- group delete <uuid> - Delete group
- group add-member <uuid> <user-uuid> - Add single member (incremental)
- group remove-member <uuid> <user-uuid> - Remove single member (incremental)

The add-member and remove-member commands fetch current members, modify the list,
and update atomically - providing a better UX than replacing the entire list.

Backend API endpoints used:
- POST /api/groups - Create group
- GET /api/groups/{uuid} - Get group details
- PUT /api/groups/{uuid} - Update members
- DELETE /api/groups/{uuid} - Delete group

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

main.go

@@ -95,6 +95,8 @@ func main() {
 			} else {
 				client.handleUserList(args)
 			}
+		case "group":
+			client.handleGroup(args)
 		default:
 			fmt.Println(red("Unknown command:"), cmd)
 			fmt.Println("Type 'help' for available commands")