fix: ignore system DMs in reactor — prevent stalemate notification cascade

The StaleWorker sends DMs from 'system' to all channel members when
workflow messages are stuck in 'proposed' state. These DMs were
triggering reactive agent runs, which couldn't action the stale
messages, burning daily budget on wasted K8s Jobs.

Now: reactor silently ignores all messages from 'system' sender.
System notifications are for human review, not agent action.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/reactor/reactor.go

@@ -75,6 +75,13 @@ func (r *Reactor) Dispatch(ctx context.Context, event dispatcher.MessageEvent) e
 
 // evaluateTrigger runs the decision chain for a single agent.
 func (r *Reactor) evaluateTrigger(ctx context.Context, agentName string, event dispatcher.MessageEvent) error {
+	// 0. Ignore system messages — stalemate worker notifications, retention warnings,
+	// and other automated DMs should NOT trigger reactive runs. They're notifications
+	// meant for the human owner, not actionable work for agents.
+	if event.FromAgent == "system" {
+		return nil
+	}
+
 	// 1. Get agent config
 	agent, err := r.agentStore.GetAgentByName(ctx, agentName)
 	if err != nil {