- Add TaskInput schema with all fields per architecture (id, name, dependsOn,
categorical fields as Optional(Nullable(...)), metadata fields)
- Add DependencyEdge schema with from, to, qualityRetention fields
- Re-export Nullable helper from task.ts for convenience
- Add type aliases: TaskInput, DependencyEdge via Static<typeof>
- Add 49 tests covering validation, nullable fields, edge cases, type correctness