A mutation report on changed lines #155

What to build

A pull request shows which mutants survive on the lines it changes, so a reviewer sees a test that runs code without checking it. A reusable workflow runs gremlins for Go and Stryker's incremental mode for TypeScript, scoped to the diff, and reports without failing the build. identity is the first repo to use it.

Part of the estate testing standard: spec 007, testing standards.

Acceptance criteria

• A reusable workflow takes the changed files of a pull request and runs gremlins (Go) and Stryker incremental (TypeScript) on them
• Surviving mutants are reported on the pull request; the job never fails the build
• identity calls it; a pull request with a deliberately weak test shows a surviving mutant
• Billing: the job runs once per pull request, in one job with steps, per the workspace CLAUDE.md

Blocked by

• None (can start immediately)