Definition
The word means the tack that connects a horse to what it pulls. The harness provides no pulling force of its own. It sets the direction, distributes the load and makes the power usable. The image holds unusually well for what it describes.
A harness is everything around the language model itself. The loop that runs it again until the task is done. The definitions of the tools it may use. The limits on what it is allowed to do, and what requires a human to approve. The assembly of context: what goes into the window and what stays out. The logging and evaluation that let you say whether it got better or worse after a change.
It is worth noting who owns what. The model can be swapped out, and is swapped out, often. The harness is what you actually build, and it is where the difference between a demo and something that survives production lies. Two companies using the same model can get wildly different results, and the difference is almost always here.
Norwegian has no single word covering all of it. Styringsrammeverk, control framework, is the precise one when talking about the infrastructure that runs and evaluates the agent. Testrammeverk, test framework, covers the measuring part. Kjøremiljø or agentmiljø, runtime or agent environment, fits when thinking of the sandbox it operates in. In informal shop talk Norwegian developers usually just say harness, which is fine spoken. In documentation, pick the Norwegian word that matches the part you actually mean.