[ExtraToast]

The mutation report from #157 had not yet run against a real repository. Running both tools on identity showed four things wrong, and one that made a broken run look like a perfect one.

• gremlins found nothing. Given ./..., gremlins v0.6.0 reports "No results to report" even when mutants exist on the changed lines. Given the module root ., --diff selects exactly the changed lines' mutants. Checked on identity with a one-line change: two mutants, both on that line.
• gremlins timed every mutant out. At its default timeout coefficient, all six mutants in identity's Launcher service timed out and efficacy read 0%. At --timeout-coefficient 40 (as grimoire uses) all six are killed and efficacy reads 100%. gremlins first gathers coverage over the whole module (about 10 minutes for identity), which fits the job's 30-minute tier.
• gremlins' binary path. It installs into GOBIN when mise sets one, so the workflow looks there first.
• Stryker found no test runner. Under pnpm, Stryker does not discover its plugins ("Cannot find TestRunner plugin "vitest""), so the workflow names --plugins @stryker-mutator/vitest-runner.
• Stryker has no flag for the JSON report's path. It always writes reports/mutation/mutation.json, which the workflow now copies.
• A crash read as success. A tool that crashes leaves no report, and a missing report counted as "Every mutant on the changed lines was caught by a test." The workflow now passes each failed tool's step outcome as --failed. The comment then says "Stryker did not finish, so its mutants are not listed: see the job log." and no longer claims every mutant was caught.

Part of JorisJonkers-dev/workspace#128, for #155.

Checks

• tests/test_mutation_report.py: 15 tests, 100% line and branch coverage. The new test covers a tool that did not finish.
• Locally on identity: Stryker with the plugin flag writes the JSON report for useLauncher.ts (survivors listed). gremlins with . and --diff selects the changed line's two mutants.
• actionlint passes.