feat(signals): slop signal — missing test evidence

Parent roadmap: #525
Parent epic: #530

## Goal
Implement the missing-test-evidence slop signal.

## Reuse
`hasLocalTestEvidence`/`isTestPath` in `src/signals/test-evidence.ts`.

## Acceptance
Code-only diffs with no tests raise the signal; test-bearing diffs don't.

## Testing
`npm run test:ci`, 97%+ coverage. Public/private output boundary preserved; deterministic where applicable.
