fix: remove nextRun recalculation from getAgents() that blocked cron

The getAgents() function was recalculating nextRun to future dates when
it found past values. This prevented the cron scheduler from ever
finding due agents (nextRun <= now was always false since getAgents
had already pushed it to the future).

Also fix toast polling: use null sentinel for agents without initial
actions so first execution is still detected. Limit cron to 3 agents
per cycle and add logging.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

memento-note/app/(main)/agents/agents-page-client.tsx

@@ -91,16 +91,15 @@ export function AgentsPageClient({
   }, [])
 
   // Track latest action per agent to detect new executions
-  const prevActionsRef = useRef<Record<string, string>>({})
+  // null = agent tracked with no initial actions, undefined = not tracked yet
+  const prevActionsRef = useRef<Record<string, string | null>>({})
 
   // Poll every 30s to detect agent executions and show toast notifications
   useEffect(() => {
-    // Initialize tracking from initial data
+    // Initialize tracking from initial data — use null for agents without actions
+    // so we can still detect their FIRST execution
     for (const agent of initialAgents) {
-      const lastAction = agent.actions[0]
-      if (lastAction) {
-        prevActionsRef.current[agent.id] = lastAction.id
-      }
+      prevActionsRef.current[agent.id] = agent.actions[0]?.id ?? null
     }
 
     const interval = setInterval(async () => {
@@ -112,8 +111,11 @@ export function AgentsPageClient({
         if (!lastAction) continue
 
         const prevId = prevActionsRef.current[agent.id]
-        if (prevId && prevId !== lastAction.id) {
-          // New action detected
+        // undefined = agent not in initial list (created by someone else, skip)
+        if (prevId === undefined) continue
+
+        if (prevId !== lastAction.id) {
+          // New execution detected (first action ever, or new run)
           if (lastAction.status === 'success') {
             toast.success(t('agents.toasts.autoRunSuccess', { name: agent.name }))
           } else if (lastAction.status === 'failure') {