Use when doing any non-trivial engineering work - implementing, debugging, refactoring, configuring, operating, or investigating why a system misbehaves - or any change where being wrong has a cost. Encodes the evidence-over-theory discipline, the hard safety rules, and the pre-change checkpoint. Use whenever code, data, or infrastructure is about to change or must be understood before it can, even if the task looks routine or is only "find out why". Foundation for the sibling skills.