In April, I arrived on a visa-free travel basis and have a valid work permit for the farm until the end of December. What happens after that time when I run out of my stay entitlement?

If you were staying in Poland on 14 March 2020 under the visa-free regime and the date of your legal stay under this regime ends during an epidemic state or epidemic emergency, you may legally stay in Poland until the 30th day following the date of cancellation of the last state. Your legal stay will be extended by law, and you do not have to complete any additional formalities.

If you want to stay in Poland longer and your purpose of stay is seasonal work, apply for a temporary residence permit due to seasonal work. Details of the procedure can be found HERE.