What is a task, and what states can it be in?

A task is the unit of work: a brief, the agent that runs it, the files it uses, and everything that happens while it runs. Tasks are planned, active, completed, or failed, and you can cancel a running one. Every run keeps its record (what it read, which tools it called, what it produced, and any approvals), and a follow-up message continues the same task rather than starting over.