Chat, slash commands, and completion

The official cyberm_chat package is documented in docs/chat.md. It provides English-only UI text, authenticated slash-command dispatch, automatic suggestion discovery, history, and keyboard completion.

Quick usage:

T                 open chat
Enter             send or execute
Escape            close
Arrow Up/Down     select a suggestion while typing /
Tab               complete the selected command

Slash commands use the same server-side RegisterCommand and ACL path as the CyberM terminal; they are not chat messages and are never broadcast to other players.

Built-in utility commands include:

/id    show the temporary session player ID
/pos   copy the current world position as Lua
/rot   copy the current quaternion and yaw as Lua

/pos and /rot write only on the requesting client and display their result in chat and through the shared notification UI. See the clipboard guide for the API, permission, output formats, limits, and failure reasons.