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/analysis/parallel-groups.md

@@ -0,0 +1,38 @@
+---
+id: analysis/parallel-groups
+name: Implement parallelGroups analysis function
+status: pending
+depends_on:
+  - graph/construction
+  - graph/queries
+scope: narrow
+risk: low
+impact: component
+level: implementation
+---
+
+## Description
+
+Implement `parallelGroups(graph: TaskGraph): string[][]` in `src/analysis/index.ts` or a dedicated module. This returns groups of tasks that can be executed concurrently — tasks at the same topological depth. Uses `graphology-dag.topologicalGenerations`.
+
+## Acceptance Criteria
+
+- [ ] `parallelGroups` returns `string[][]` where each inner array is a generation of tasks at the same depth from sources
+- [ ] Uses `graphology-dag.topologicalGenerations()` for the generation computation
+- [ ] Tasks with zero prerequisites are in the first group
+- [ ] Throws `CircularDependencyError` if the graph is cyclic (delegated to `topologicalGenerations` behavior)
+- [ ] Works on disconnected graphs (each connected component sorted independently, then merged by depth)
+- [ ] Unit tests: linear chain (each group size 1), diamond graph, disconnected components
+
+## References
+
+- docs/architecture/api-surface.md — parallelGroups signature
+- docs/architecture/graph-model.md — parallel groups definition
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion