Removes 6 files audited as fully unused: StageExecutor interface (shadowed
by kernel), CyclePolicyResolver and PolicyValidation (deferred cycle policy),
ApprovalRoutes (never registered), TuiNavigation (superseded by reducer),
and ReplayContractTest abstract class (no concrete subclasses).
ArtifactRelationshipType and CyclePolicy were audited but retained —
both are actively referenced by production code.