Delete a managed specialist agent.

Removes the definition file from the managed directory and refreshes the roster, so the agent is no
longer available for delegation. Only agents that were created through these tools can be deleted;
an owner-authored agent in [orchestration].directory is never touched, and the result reports
`ok: false` with an error when the name is not a managed agent.

Deleting an agent does not cancel tasks that are already running on it.
