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/graph/taskgraph-class.md

@@ -0,0 +1,45 @@
+---
+id: graph/taskgraph-class
+name: Implement TaskGraph class skeleton with graphology DirectedGraph
+status: pending
+depends_on:
+  - schema/enums
+  - schema/input-schemas
+  - schema/graph-schemas
+  - error/error-hierarchy
+  - setup/project-init
+scope: moderate
+risk: medium
+impact: phase
+level: implementation
+---
+
+## Description
+
+Create the `TaskGraph` class in `src/graph/index.ts` that wraps `graphology.DirectedGraph`. This is the data class that holds the graph instance and provides the foundation for construction, mutation, and query methods. At this stage, implement the constructor, `raw` getter, and the overall class structure. Actual construction and analysis methods come in dependent tasks.
+
+## Acceptance Criteria
+
+- [ ] `src/graph/index.ts` exports `TaskGraph` class
+- [ ] Constructor creates an internal `graphology.DirectedGraph` with options `{ type: 'directed', multi: false, allowSelfLoops: false }`
+- [ ] `get raw(): Graph` returns the underlying graphology instance
+- [ ] Constructor accepts optional `TaskGraphSerialized` for initializing from serialized data (delegates to `fromJSON` pattern)
+- [ ] Class stores edge key format: `${source}->${target}` (per ADR-006)
+- [ ] No parallel edges constraint enforced by `multi: false` graph option
+- [ ] No self-loops constraint enforced by `allowSelfLoops: false` graph option
+- [ ] Internal `_edgeKey(source, target): string` method producing deterministic keys
+- [ ] Re-exported from `src/index.ts`
+
+## References
+
+- docs/architecture/api-surface.md — TaskGraph class API
+- docs/architecture/graph-model.md — construction paths, edge direction, constraints
+- docs/architecture/decisions/006-deterministic-edge-keys.md — edge key format
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion