[Feature]: Add a configurable session idle timeout that automatically resets conversation context

Problem / Motivation

When a user returns to a channel after a long break (e.g., the next morning), the assistant still operates within the old conversation context. This
causes the assistant to reference outdated or irrelevant prior messages, leading to confusing or incorrect responses. Users must remember to manually
type /new to reset — which is easy to forget.

This affects all users on long-running deployments (DingTalk, Feishu, Discord bots, etc.) where conversations happen asynchronously across sessions.

Proposed Solution

Introduce a configurable session_idle_timeout option (e.g., in config.yaml) at the channel or global level. When a new message arrives from a user, the
backend checks the timestamp of their last message in that channel. If the elapsed time exceeds the configured threshold, the conversation context is
automatically cleared before processing the new message — identical in effect to the user issuing /new.