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/export.md

@@ -0,0 +1,36 @@
+---
+id: graph/export
+name: Implement TaskGraph export methods (export, toJSON)
+status: pending
+depends_on:
+  - graph/taskgraph-class
+scope: single
+risk: trivial
+impact: component
+level: implementation
+---
+
+## Description
+
+Implement the `export()` and `toJSON()` methods on `TaskGraph`. These wrap graphology's `export()` to produce `TaskGraphSerialized` output.
+
+## Acceptance Criteria
+
+- [ ] `export(): TaskGraphSerialized` — wraps `graph.export()` and validates the output conforms to the `TaskGraphSerialized` schema
+- [ ] `toJSON(): TaskGraphSerialized` — alias for `export()` (enables `JSON.stringify(graph)` to work)
+- [ ] Exported data includes all node attributes and edge attributes (including `qualityRetention`)
+- [ ] Round-trip: `TaskGraph.fromJSON(graph.export())` produces an equivalent graph
+- [ ] Unit test: create graph, add tasks/edges, export, round-trip through fromJSON, verify equivalence
+
+## References
+
+- docs/architecture/api-surface.md — export/toJSON methods
+- docs/architecture/schemas.md — TaskGraphSerialized schema
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion