Task completion notification

Key features

• Dialogue task completion reminder (agent’s turn ends)
• Pure client listeningsubscribe to ctx.sessions.list, do diff on two adjacent snapshots, detect turn completion, job completion, pendingInteraction (requires review) and lastAgentError (failure).
• Background task completion reminder (end of background command/subagent job)
• Reminder when review is required (approval/plan review/question)
• Failure reminder (dialogue error/background task failed or terminated)