fa

Add a new in-game chat slash command (e.g. `/foo <arg>`) to the FAF chat system. Use when the user asks to add, register, or scaffold a new chat slash-command, or extend the `/help` listing with a new entry. Creates the command file under `lua/ui/game/chat/commands/builtin/`, wires it into `ChatController.RegisterBuiltinCommands`, and follows the parser/Accept/Execute split.