Is Atlassian Rovo free?
No, but it is included. Rovo has no separate per-seat charge for customers on a paid Standard, Premium or Enterprise Cloud plan of Jira, Confluence or Jira Service Management. The Atlassian pricing page states that Rovo, including Search, Chat, and Studio apps as well as agents, is available to those customers and that no separate Rovo purchase is required. So there is nothing extra to buy, and for a team already paying for Atlassian the marginal cost of turning Rovo on is zero.
The word doing the work is "included" rather than "unlimited". Rovo usage is governed by an allowance, and the Atlassian support documentation is explicit that Rovo usage is governed by your subscription tier and the number of users on your base subscription. The pricing page also refers to two distinct quotas of Rovo usage. In other words the price is capped, and so is the product.
Free trials do exist. You can evaluate Rovo through a trial of a Standard, Premium or Enterprise plan of Jira, Confluence or Jira Service Management, and Rovo Dev has its own 30-day trial. What does not exist is a permanent free Rovo tier for a company that is not already an Atlassian customer.