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>

cmd_system.go

@@ -52,6 +52,14 @@ User Management:
   user get <uuid>            - Get user details
   user create <nickname>     - Create new user (admin only)
 
+Group Management:
+  group create <name> [members] - Create new group
+  group get <uuid>           - Get group details
+  group update <uuid> <members> - Replace all group members
+  group delete <uuid>        - Delete group
+  group add-member <uuid> <user-uuid> - Add member to group
+  group remove-member <uuid> <user-uuid> - Remove member from group
+
 System:
   help                       - Show this help
   exit, quit                 - Exit shell