fix(ctxmgr): normalize dismissed_unread_ids for Postgres array insert

Ensure nil dismissed ID slices serialize as empty bigint arrays and
preallocate compressDismissed output to avoid NULL write failures.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/ctxmgr/chat_unread.go

@@ -328,13 +328,17 @@ func (m *ContextManager) markChatReadPG(ctx context.Context, req MarkChatReadReq
 	pos.LastReadMessageID, pos.DismissedUnreadIDs = compressDismissed(pos.LastReadMessageID, pos.DismissedUnreadIDs, futureIDs)
 	pos.UpdatedAt = time.Now()
 
+	if pos.DismissedUnreadIDs == nil {
+		pos.DismissedUnreadIDs = []int64{}
+	}
+
 	_, err = tx.Exec(ctx, `
 		INSERT INTO chat_read_positions (session_id, user_id, last_read_message_id, dismissed_unread_ids, updated_at)
-		VALUES ($1, $2, $3, $4, $5)
+		VALUES ($1, $2, $3, COALESCE($4, '{}'::bigint[]), $5)
 		ON CONFLICT (session_id, user_id)
 		DO UPDATE SET
 			last_read_message_id = EXCLUDED.last_read_message_id,
-			dismissed_unread_ids = EXCLUDED.dismissed_unread_ids,
+			dismissed_unread_ids = COALESCE(EXCLUDED.dismissed_unread_ids, '{}'::bigint[]),
 			updated_at = EXCLUDED.updated_at
 	`, pos.SessionID, pos.UserID, pos.LastReadMessageID, pos.DismissedUnreadIDs, pos.UpdatedAt)
 	if err != nil {
@@ -375,7 +379,7 @@ func compressDismissed(baseline int64, dismissed []int64, futureMsgIDs []int64)
 		}
 	}
 
-	var newDismissed []int64
+	newDismissed := make([]int64, 0, len(dMap))
 	for id := range dMap {
 		if id > baseline {
 			newDismissed = append(newDismissed, id)