Implement unified group‑chat context and optional GROUP_ADMINS setting

[s-nagaev]

Goal

Enable Chibi to maintain a single, unified conversation context in group chats and introduce an optional GROUP_ADMINS setting to control who can change the model and reset the chat.

Motivation

• In group chats, Chibi currently treats each mention or reply as a separate conversation, losing context between participants.
• Defining group‑bot administrators allows granular control over critical commands (e.g. changing the LLM or resetting history) and prevents misuse.

Tasks

1. Unified group context

   • Detect messages in a group chat where Chibi is addressed (mentions or replies).
   • Append all such messages into one shared conversation history per chat (instead of per‑user).
   • Ensure replies and mentions both feed into and read from this global context.

2. GROUP_ADMINS setting

   • Introduce an env‑var or config key GROUP_ADMINS (e.g. list of user IDs).
   • Only users in GROUP_ADMINS may use /set_model and /reset commands in group chats.
   • Others receive a “permission denied” message if they attempt those commands.

Reference

Similar implementation in Hiroshi: s-nagaev/hiroshi#16