feat(signals): slop signal — no linked issue without rationale

[JSONbored]

Parent roadmap: #525
Parent epic: #530

Goal

Implement the no-linked-issue-without-rationale signal.

Reuse

hasClearNoIssueRationale (engine.ts:3895) + preflight missing_linked_issue.

Acceptance

Raises when no issue and no rationale and repo not issue-discovery lane.

Testing

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