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/error/error-hierarchy.md

@@ -0,0 +1,43 @@
+---
+id: error/error-hierarchy
+name: Implement typed error class hierarchy
+status: pending
+depends_on:
+  - setup/project-init
+scope: narrow
+risk: trivial
+impact: phase
+level: implementation
+---
+
+## Description
+
+Implement the custom error classes in `src/error/index.ts` per [errors-validation.md](../../../docs/architecture/errors-validation.md). All errors extend `TaskgraphError` which extends `Error`. Each subclass adds typed fields for programmatic error recovery.
+
+## Acceptance Criteria
+
+- [ ] `src/error/index.ts` exports:
+  - `TaskgraphError extends Error` — base class
+  - `TaskNotFoundError extends TaskgraphError` with `taskId: string` field
+  - `CircularDependencyError extends TaskgraphError` with `cycles: string[][]` field
+  - `InvalidInputError extends TaskgraphError` with `field: string` and `message: string` fields
+  - `DuplicateNodeError extends TaskgraphError` with `taskId: string` field
+  - `DuplicateEdgeError extends TaskgraphError` with `prerequisite: string` and `dependent: string` fields
+- [ ] Each error class sets `this.name` to the class name
+- [ ] Each error class properly extends the prototype chain (`Object.setPrototypeOf(this, new.target.prototype)`)
+- [ ] `InvalidInputError` supports construction from TypeBox `Value.Errors()` output (receives structured field/path/value info)
+- [ ] `CircularDependencyError` receives `string[][]` where each inner array is an ordered cycle path
+- [ ] Unit tests verifying: correct `instanceof` chain, field access, `.name` property, error messages
+
+## References
+
+- docs/architecture/errors-validation.md — error types, when each is thrown
+- docs/architecture/api-surface.md — error documentation on specific methods
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion