E2E scenarios in markdown, test scripts as build artifact

Test scenarios live as numbered markdown files per feature area, with preconditions and expected results - readable, reviewable, diffable by anyone. The runnable scripts are generated from them, committed so they can be inspected when something fails, and regenerated only when the source scenario actually changed.

The rule that makes it hold: any task that changes user-facing behaviour updates the scenario and regenerates its script in the same commit. Stale generated scripts are the single biggest source of noisy full-suite failures, and they are the thing an agent will silently skip unless you make it explicit.