Scope both mutation engines to changed production files

[jonbaldie]

Parent

#21

What to build

Make a source-changing pull request run both Mutasaurus and Stryker against the identical production-file set selected from its Git diff. Preserve each engine's existing test command, mutation behavior, and per-file score enforcement while reducing work to the files the PR can affect.

This tracer bullet makes the selected-path mode observable from a real source diff through both engines and their quality gates.

Acceptance criteria

• Added and modified production TypeScript files are passed to both engines as the identical path set.
• Renamed and copied files are mutated under their head paths.
• Deleted files and changed paths outside production source are not passed to either engine.
• Mutasaurus no longer relies only on a divergent static target list for PR-scoped execution.
• Stryker's broad default pattern is safely overridden for PR-scoped execution.
• Selected paths remain data and are not interpolated into executable shell fragments.
• Existing engine versions, test commands, operators, concurrency, and per-file threshold enforcement remain unchanged.
• A selected file below the existing threshold fails its job; selected files meeting the threshold pass.
• CI logs show the common selected path set before each engine runs.
• Integration coverage proves both engine entry points receive the same selection.
• A real source-change tracer-bullet PR demonstrates reduced mutation scope with both checks green.

Blocked by

• Skip both mutation engines for irrelevant PR diffs #23