Built-in executor for "agent" tasks: drives a dedicated sub-session conversation. The sub-session is a real `ISessionGrain` (key = the task's "userId/sessionId/taskId"), so it gets the full harness, transcript, and history machinery. It inherits the parent session's workspace, tools, and runtime, and is marked `Kind = "task"` so it runs the (otherwise async) agent inline instead of recursively spawning another task.