- HeartbeatMonitor.executeHeartbeat() now checks if resolved task is done/archived
and exits before session creation with reason 'task_closed'
- When stale task came from persisted agent.taskId, clears the linkage to prevent
repeated stale activations
- HeartbeatTriggerScheduler.watchAssignments() now skips assignment callback dispatch
when assigned task is done/archived (when taskStore is available)
- Added comprehensive tests for stale-task activation guard behavior
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