Harness fundamentals — the agent loop, what the harness provides

The harness is everything wrapping the model. Students learn the core agent loop
(the model proposes an action, the harness executes it and feeds back the result,
repeat) and the services a harness must provide around that loop: filesystem and
shell access, the tool layer, permission handling, and state. This is the
conceptual anchor for the entire module — every later topic is a component of, or a
variation on, the harness.