- Mentions were doing a bunch of mutations; not just uses derived state
- no need for a separate `ClientMention`
- added `extractInputHistory` for shared logic
- tests made by claude
The framework may emit messages which do not have a time stamp.
We tried to unconditionally convert the time field, fix that.
The Msg constructor replaces falsey time fields with the current
date so we can also remove the duplication from that codepath.
