This patch introduces a new `baibot_conversation_start_time_utc`
variable which indicates the time the conversation got started.
Using `baibot_now_utc` is still possible, but given that the current
time is a moving target, its use is in conflict with prompt caching.
Because the new `baibot_conversation_start_time_utc` prompt variable
is a more reasonable default, we're now using it in all sample configs.
