Wire mutation testing into CI

[PAMulligan]

Problem

@stryker-mutator/core, @stryker-mutator/typescript-checker, and @stryker-mutator/vitest-runner are declared in devDependencies, but no GitHub Actions workflow invokes Stryker. Mutation score is referenced in Phase 8 (Quality Gate) but isn't actually being measured.

Acceptance criteria

• stryker.conf.json (or .mjs) at the repo root with sensible defaults
• CI workflow .github/workflows/mutation.yml that runs mutation testing (nightly or on-demand to avoid PR latency)
• Mutation score threshold defined in pipeline.config.json (opt-in per the README)
• README updated with how to run locally

Milestone

v2.0.0