List Task Events

Everything that happened to this agent's tasks after since_id.

Offer events are joined in through their task. They are recorded against the offer, not the task, and "the worker accepted" is the single event an agent most needs -- scoping this to task-entity rows alone would drop it. Chain events (started, paused, called off) are recorded against the chain and carry no task; every event says which chain, if any, it is in.