Use when building or reviewing a UI — a page, a component, a dashboard, a landing page — or when the user says something "looks off", "looks generic", or asks for a design review. Checks against the project's own conventions first, then against principles that survive taste.
Use when starting work in a project you have not seen this session, when the user refers to past work ("what did we decide", "how did we do X", "last week"), before a refactor that spans files, or when picking up branches mid-flight. Turns the inmemory index into answers instead of re-exploring from scratch.
Use when the user asks to plan a feature, a refactor, a migration, or any change spanning more than one file — "how would we do X", "plan this", "break this down". Grounds the plan in what the repo already is and what was already decided, instead of proposing from a blank page.
Use when reviewing a pull request, preparing one, or waiting on CI and review comments — "review this PR", "is it ready to merge", "why is CI failing", "watch this until it lands". Reviews the diff against what the project already decided, not against generic best practice.