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/review/schemas-and-errors.md

@@ -0,0 +1,49 @@
+---
+id: review/schemas-and-errors
+name: Review schema, enum, and error implementations for consistency
+status: pending
+depends_on:
+  - schema/enums
+  - schema/input-schemas
+  - schema/graph-schemas
+  - schema/result-types
+  - schema/numeric-methods-and-defaults
+  - error/error-hierarchy
+scope: narrow
+risk: low
+impact: phase
+level: review
+---
+
+## Description
+
+Review the schema and error layer implementations before building the graph and analysis layers on top. This is a critical checkpoint because everything downstream depends on these types being correct and consistent with the architecture docs.
+
+## Acceptance Criteria
+
+- [ ] All TypeBox schemas match [schemas.md](../../../docs/architecture/schemas.md) exactly
+- [ ] All `Static<typeof>` type aliases correctly derived — no manual type definitions
+- [ ] Nullable helper used consistently in TaskInput (not in TaskGraphNodeAttributes)
+- [ ] Enum values match DB/frontmatter conventions exactly
+- [ ] Numeric method tables match spec tables exactly
+- [ ] `resolveDefaults` correctly separates "nullable categorical→default" from "label-only nullable→stays nullable"
+- [ ] Error class hierarchy is correct: all extend TaskgraphError, all have proper `name` and typed fields
+- [ ] `InvalidInputError` can be constructed from `Value.Errors()` output
+- [ ] `CircularDependencyError.cycles` type is `string[][]`
+- [ ] No Zod, no gray-matter, no js-yaml in any dependency
+- [ ] `package.json` lists only approved dependencies
+- [ ] All tests pass
+
+## References
+
+- docs/architecture/schemas.md
+- docs/architecture/errors-validation.md
+- docs/architecture/frontmatter.md — supply chain constraints
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion