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/cost-benefit/workflow-cost.md

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+---
+id: cost-benefit/workflow-cost
+name: Implement workflowCost orchestration function
+status: pending
+depends_on:
+  - cost-benefit/dag-propagation
+  - graph/queries
+scope: moderate
+risk: medium
+impact: component
+level: implementation
+---
+
+## Description
+
+Implement `workflowCost(graph: TaskGraph, options?: WorkflowCostOptions): WorkflowCostResult` in `src/analysis/cost-benefit.ts`. This orchestrates the per-task EV calculations, handles DAG propagation, and enriches results with `taskId` and `name` from the graph.
+
+## Acceptance Criteria
+
+- [ ] `workflowCost` accepts `WorkflowCostOptions` with optional: `includeCompleted`, `limit`, `propagationMode`, `defaultQualityRetention`
+- [ ] Default propagation mode: `"dag-propagate"` per ADR-004
+- [ ] Default `defaultQualityRetention`: 0.9
+- [ ] Each task in result includes: `taskId`, `name`, `ev`, `pIntrinsic`, `pEffective`, `probability` (= `pEffective`), `scopeCost`, `impactWeight`
+- [ ] `totalEv`: sum of all task EVs (excluding completed tasks from output when `includeCompleted: false`)
+- [ ] `averageEv`: `totalEv / tasks.length`
+- [ ] `propagationMode`: reflected in result
+- [ ] When `includeCompleted: false`: completed tasks excluded from `tasks` array but remain in propagation chain with p=1.0
+- [ ] When `includeCompleted: false`: only `"completed"` status triggers exclusion; `"failed"` and `"blocked"` are always included
+- [ ] When `limit` is set: returns at most `limit` tasks (sorted by EV descending? or topological order? spec says "limits the number of tasks in the result" — use topological order with limit)
+- [ ] Throws `CircularDependencyError` if graph is cyclic
+- [ ] Unit tests: full workflow cost calculation, independent vs dag-propagate comparison, excludeCompleted scenarios, limit behavior
+
+## References
+
+- docs/architecture/cost-benefit.md — workflow cost, skip-completed semantics
+- docs/architecture/api-surface.md — workflowCost signature, WorkflowCostOptions
+- docs/architecture/decisions/004-workflow-cost-dag-propagation.md — ADR-004
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion