Skip both mutation engines for irrelevant PR diffs

[jonbaldie]

Parent

#21

What to build

Wire the shared target selector through the mutation workflow so a pull request containing only documentation, agent guidance, or editor/project tooling completes both Mutasaurus and Stryker as explicit successful skips. Contributors should see why mutation was skipped, while ordinary compilation and tests remain required before the skip decisions are reached.

This is the first end-to-end tracer bullet through Git comparison, CI orchestration, both engine entry points, statuses, and human-readable logs.

Acceptance criteria

• Both mutation jobs consume the shared selector rather than implementing separate Git-diff logic.
• A docs-only PR completes the Mutasaurus and Stryker statuses successfully without starting mutation execution.
• An agent-guidance-only or project-tooling-only PR behaves the same way.
• Each skipped job logs the comparison base, clean-skip reason, and that no mutation engine was invoked.
• Compilation and the ordinary test suite remain prerequisites for mutation jobs.
• An empty selected set is handled as an explicit skip, never passed accidentally to an engine as a broad/default target pattern.
• A real CI-facing process test verifies exit status and log output without mocks.
• Existing full-run behavior remains available for modes other than clean skip.

Blocked by

• Build a safe PR mutation-target selector #22