mutation-testlisted

# Mutation testing A green test run is the **null result**, not evidence. A dead assertion and a live one produce identical output, so "it passed" discriminates nothing. This skill establishes the one thing that does discriminate: **break the behaviour and watch it go red.** Work is not done when it passes. It is done when it has been seen to fail for the right reason. ## Scope, and what this version does not do This version runs **one claim at a time, by hand**. There is no batch runner, no coverage map, and no scripted scope resolution — see [Not in this version](#not-in-this-version). If you were handed a PR or a diff and asked whether it is pinned by its suite, say so plainly and pick the claims worth checking individually rather than implying a sweep happened. ## Rules - **Break the source, never the test.** Mutating the test proves only that the test tests itself. - **Never mutate test files.** - **Copy the file before you touch it, and restore from that copy.** Not from git. The files this skill is pointed at are usually the ones just written, so they hold uncommitted work — `git checkout`, `git restore` and `git stash` destroy it rather than restoring it. `cp <file> <file>.mtbak`, mutate, then `mv` it back. - **Mutate the file where the tests actually load it from — in place.** The tempting shortcut is to mutate a copy instead, so the real tree stays clean: a `git worktree`, a scratch checkout, a second clone. By default that does not work, and it