mutation-testing

Run diff-scoped mutation analysis and report surviving mutants. Restores the code under test and fails if tracked source is not byte-identical; writes no test. One mutant per changed line, then a fresh-context reviewer judges productive versus arid versus equivalent; ranks files by oracle gap and hands survivors to the test-authoring lane. Use when: the user asks to run mutation testing or wants a mutation score for a change ('run mutation testing'), doubts a suite whose coverage report looks healthy ('my coverage is high but I do not trust it'), asks whether the tests actually check the code, asks to audit test quality, or asks for the survivors persisted for the fix pass; after tests go green and before review. Flags: `--full` (whole configured scope, not the diff), `--paths <globs>`, `--max <n>`, `--no-suppress` (report suppressed arid mutants too), `--persist-findings` (write the survivors as a findings file the review fix pass consumes).

Answers mutation-testing questions from the primary literature (DeMillo/Lipton/Sayward, Jia & Harman, Google's ICSE-SEIP papers, and the Stryker/PIT/Infection tool docs), producing WHY reasoning about what a surviving mutant means. Use when: the user asks what mutation testing is or whether it is worth doing, asks how the score relates to coverage ('mutation score vs coverage', 'what is test strength'), asks what a mutant's state or disposition means ('equivalent mutant', 'what is an arid node'), asks which operators to use or why a score is low, asks whether to gate a build on the score, or names the underlying theory ('coupling effect', 'competent programmer hypothesis'); not for HOW to run a mutation tool in the project (use `/mutation-testing:setup` and `/mutation-testing:audit`).

Verify and configure the mutation-testing plugin for this repository. check inspects the ecosystem's mutation tool (installed, runnable, supported test runner), the baseline suite's health, and the tracked .claude/mutation-testing.md config across its merge layers, read-only; apply detects the ecosystem, installs or names the tool, interviews for the diff target and operator set, and writes the config plus an empty arid-node suppression record. Use when: 'set up mutation testing', 'is mutation testing configured', 'which mutation tool for this repo', 'mutation-testing setup', or the audit skill reports missing config or an unavailable tool. Re-runnable. Safe to invoke again to reconfigure.