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/graph-complete.md

@@ -0,0 +1,53 @@
+---
+id: review/graph-complete
+name: Review TaskGraph class implementation for correctness and API compliance
+status: pending
+depends_on:
+  - graph/construction
+  - graph/mutation
+  - graph/queries
+  - graph/subgraph-and-validation
+  - graph/export
+  - review/schemas-and-errors
+scope: moderate
+risk: medium
+impact: phase
+level: review
+---
+
+## Description
+
+Review the TaskGraph class implementation before building analysis functions on top of it. The graph layer is the foundation for all analysis — incorrect behavior here propagates everywhere.
+
+## Acceptance Criteria
+
+- [ ] Construction methods match [graph-model.md](../../../docs/architecture/graph-model.md) error handling table exactly:
+  - `fromTasks`: silent orphan nodes, `DuplicateNodeError`, idempotent duplicate edges
+  - `fromRecords`: `TaskNotFoundError` for dangling edges, `DuplicateNodeError`, `DuplicateEdgeError`
+  - `fromJSON`: validated against schema, orphans preserved
+  - `addTask`: `DuplicateNodeError`
+  - `addDependency`: `TaskNotFoundError`, `DuplicateEdgeError`
+- [ ] Edge direction is prerequisite→dependent throughout (matches Rust CLI convention)
+- [ ] Deterministic edge keys `${source}->${target}` used via `addEdgeWithKey` (ADR-006)
+- [ ] `topologicalOrder` throws `CircularDependencyError` with `cycles` populated (ADR-003)
+- [ ] `findCycles` returns actual cycle paths (not just SCCs)
+- [ ] `subgraph` returns internal-only edges (ADR-007)
+- [ ] Validation methods return arrays, never throw
+- [ ] Mutation error semantics match [errors-validation.md](../../../docs/architecture/errors-validation.md) table (no-op for remove, throws for update on nonexistent)
+- [ ] `export()`/`toJSON()` round-trips correctly
+- [ ] `raw` getter exposed, warning about direct mutation documented in code comments
+- [ ] All tests pass, including edge cases (empty graphs, single-node, cyclic graphs, disconnected components)
+
+## References
+
+- docs/architecture/graph-model.md
+- docs/architecture/api-surface.md
+- docs/architecture/errors-validation.md
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion