Adding missing liquidate collateral message command

Overseer contract must have the `liquidate-collateral` message command. The following message must be added to overseer command:
- [ ] adding the message command

Start at the overseer command entry point and trace how existing message commands are defined and dispatched. Add the missing liquidate-collateral message command consistently with the other overseer commands, then verify that the command is exposed and can be invoked successfully.

Written by the indexing model from the issue text.