Capture more data when setting an agent offline.

Presently there is a comment captured by the agent as it set itself offline, and that note include the id of the last job that ran, however it does not capture the time/date of when this action occurred.

The time and date of when an agent's status changed from/to online/offline should be captured. The comment can include the supplemental information such as "the agent" did it or it was controlled by an admin action, however I think the time/date information should be a new field.

Start by tracing where an agent changes its status to offline and where the existing offline comment records the last job ID. Check how status-change data is stored and covered by tests. Done means online/offline transitions retain the change time and date, while the existing comment can still record supplemental context.

Written by the indexing model from the issue text.