Decompose architecture into 28 atomic implementation tasks

Break the @alkdev/taskgraph architecture specs into dependency-ordered
implementation tasks across 8 component directories: setup, schema,
error, graph, analysis, cost-benefit, frontmatter, api, and review.
Each task has clear acceptance criteria referencing specific architecture
docs. Three review tasks serve as quality gates at critical junction
points (schemas-and-errors, graph-complete, complete-library). The
dependency graph is validated acyclic with 9 topological levels enabling
significant parallelism across independent work streams.

tasks/implementation/schema/input-schemas.md

@@ -0,0 +1,39 @@
+---
+id: schema/input-schemas
+name: Define TaskInput, DependencyEdge, and Nullable helper
+status: pending
+depends_on:
+  - schema/enums
+scope: narrow
+risk: low
+impact: component
+level: implementation
+---
+
+## Description
+
+Define the `TaskInput` and `DependencyEdge` input schemas in `src/schema/task.ts`, plus the `Nullable` generic helper. `TaskInput` uses `Type.Optional(Nullable(...))` for categorical fields to support both absent and explicitly-null values (YAML frontmatter distinction).
+
+## Acceptance Criteria
+
+- [ ] `src/schema/task.ts` exports `Nullable` helper: `const Nullable = <T extends TSchema>(T: T) => Type.Union([T, Type.Null()])`
+- [ ] `TaskInput` schema defined with all fields per [schemas.md](../../../docs/architecture/schemas.md):
+  - `id: Type.String()`, `name: Type.String()`, `dependsOn: Type.Array(Type.String())`
+  - Categorical fields: `Type.Optional(Nullable(TaskXxxEnum))` for status, scope, risk, impact, level, priority
+  - Metadata fields: `tags`, `assignee`, `due`, `created`, `modified`
+- [ ] `DependencyEdge` schema: `from: Type.String()`, `to: Type.String()`, `qualityRetention: Type.Optional(Type.Number({ default: 0.9 }))`
+- [ ] Type aliases derived: `type TaskInput = Static<typeof TaskInput>`, `type DependencyEdge = Static<typeof DependencyEdge>`
+- [ ] Re-exported from `src/schema/index.ts`
+
+## References
+
+- docs/architecture/schemas.md — TaskInput, DependencyEdge, Nullable definitions
+- docs/research/typebox-patterns.md — section 6 (Nullable helper pattern)
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion