Primer
A program is a set of decisions that have to stay true together. The person who made one of them will not be the person who next changes it.
What was supposed to happen. What changed. Who decided it was done.
Without answers, the only copy of "why" is whoever last touched the file. They leave, or they forget, and the next change is a guess.
A spec says what done means, in words someone else can test.
An item is one change, small enough to review and to remember.
The author holds the intent and the implementation at once. A second pair of eyes is how a mistake becomes visible.
What shipped, and who said it was done, has to outlive the conversation.
There is a fifth habit. Keep what the next person must not relearn. That is memory: the decision, the convention, the thing that already failed.
Those habits assume a person updates the tracker, remembers yesterday's review, and feels the cost of being wrong. The tracker is a place people look. The rule is followed because it is written down and because someone will notice.
An agent does not remember the last session, and it will report success. The next page is what that does to each habit.