fix(retention): add UTC calendar-day diversity gate to reinforceMemory

Implement OQ-2 decision: allow at most one reinforcement per memory
identity per UTC calendar day. Same-day reinforcement is blocked
regardless of session or interval. This prevents repetitive-task
gaming where a daily recurring task could reach MAX_COUNT=6 in hours.

Guard order: same-session → calendar-day → 1-hour → max-count
(existing guards kept as defense-in-depth)

1 hour guard is redundant within same day but preserved for
sub-hour edge cases.

src/retention.ts

@@ -108,6 +108,14 @@ export function calculateEffectiveAgeDays(
   return activeDays + dormantOverlapDays * DORMANT_DECAY_MULTIPLIER;
 }
 
+function isSameUTCCalendarDay(ts1: number, ts2: number): boolean {
+  const d1 = new Date(ts1);
+  const d2 = new Date(ts2);
+  return d1.getUTCFullYear() === d2.getUTCFullYear()
+    && d1.getUTCMonth() === d2.getUTCMonth()
+    && d1.getUTCDate() === d2.getUTCDate();
+}
+
 export function reinforceMemory(
   memory: LongTermMemoryEntry,
   sessionId: string,
@@ -117,6 +125,11 @@ export function reinforceMemory(
     return memory;
   }
 
+  // Calendar-day diversity gate (OQ-2): same UTC day = no reinforcement.
+  if (memory.lastReinforcedAt && isSameUTCCalendarDay(memory.lastReinforcedAt, now)) {
+    return memory;
+  }
+
   if (memory.lastReinforcedAt && now - memory.lastReinforcedAt < REINFORCEMENT_MIN_INTERVAL_MS) {
     return memory;
   }