feat(schema/input-schemas): define TaskInput, DependencyEdge schemas and Nullable re-export

- 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

src/schema/task.ts

@@ -1 +1,59 @@
-// TaskInput, DependencyEdge schemas
+import { Type, type Static } from "@alkdev/typebox";
+import {
+  Nullable,
+  TaskStatusEnum,
+  TaskScopeEnum,
+  TaskRiskEnum,
+  TaskImpactEnum,
+  TaskLevelEnum,
+  TaskPriorityEnum,
+} from "./enums.js";
+
+// Re-export Nullable for convenience (originally defined in enums.ts)
+export { Nullable } from "./enums.js";
+
+// --- Input Schemas ---
+
+/**
+ * Universal input shape for a task, matching the Rust `TaskFrontmatter` field set.
+ *
+ * Categorical fields use `Type.Optional(Nullable(...))` to support both:
+ * - Field absent (undefined) — key missing from YAML frontmatter
+ * - Field explicitly null — key present but set to null in YAML (e.g., `risk:`)
+ *
+ * This distinguishes "not yet assessed" from "intentionally set to null".
+ */
+export const TaskInput = Type.Object({
+  id: Type.String(),
+  name: Type.String(),
+  dependsOn: Type.Array(Type.String()),
+  status: Type.Optional(Nullable(TaskStatusEnum)),
+  scope: Type.Optional(Nullable(TaskScopeEnum)),
+  risk: Type.Optional(Nullable(TaskRiskEnum)),
+  impact: Type.Optional(Nullable(TaskImpactEnum)),
+  level: Type.Optional(Nullable(TaskLevelEnum)),
+  priority: Type.Optional(Nullable(TaskPriorityEnum)),
+  tags: Type.Optional(Type.Array(Type.String())),
+  assignee: Type.Optional(Nullable(Type.String())),
+  due: Type.Optional(Nullable(Type.String())),
+  created: Type.Optional(Nullable(Type.String())),
+  modified: Type.Optional(Nullable(Type.String())),
+});
+/** Inferred type from TaskInput schema */
+export type TaskInput = Static<typeof TaskInput>;
+
+/**
+ * Dependency edge between two tasks.
+ *
+ * `qualityRetention` models how much upstream quality is preserved:
+ * - 0.0 = no retention (full propagation of upstream failure)
+ * - 1.0 = complete retention (independent model)
+ * - default = 0.9
+ */
+export const DependencyEdge = Type.Object({
+  from: Type.String(),
+  to: Type.String(),
+  qualityRetention: Type.Optional(Type.Number({ default: 0.9 })),
+});
+/** Inferred type from DependencyEdge schema */
+export type DependencyEdge = Static<typeof DependencyEdge>;