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/construction.md

@@ -0,0 +1,62 @@
+---
+id: graph/construction
+name: Implement TaskGraph construction methods (fromTasks, fromRecords, fromJSON, addTask, addDependency)
+status: pending
+depends_on:
+  - graph/taskgraph-class
+scope: broad
+risk: high
+impact: phase
+level: implementation
+---
+
+## Description
+
+Implement the four construction methods in `src/graph/construction.ts` and integrate them as static methods on `TaskGraph`. These are the primary ways to create a graph instance from structured data. Each method has distinct semantics for edge handling, error behavior, and validation.
+
+Per [graph-model.md](../../../docs/architecture/graph-model.md), the preferred internal approach is to build a serialized graph JSON blob and call `graph.import()` for paths 1 and 2 (better performance than N individual addNode/addEdge calls).
+
+## Acceptance Criteria
+
+- [ ] `TaskGraph.fromTasks(tasks: TaskInput[]): TaskGraph`:
+  - Transforms `TaskInput[]` into node data + edge data, builds serialized blob, calls `graph.import()`
+  - Each `dependsOn` entry creates an edge with default `qualityRetention: 0.9`
+  - `dependsOn` targets not matching any task ID become orphan nodes with default attributes
+  - Duplicate task IDs throw `DuplicateNodeError`
+  - Uses `mergeNode` for idempotent node merging (same ID gets merged attributes)
+  - Duplicate `dependsOn` entries for the same pair create only one edge (idempotent via `addEdgeWithKey`)
+- [ ] `TaskGraph.fromRecords(tasks: TaskInput[], edges: DependencyEdge[]): TaskGraph`:
+  - Edges must reference tasks that exist in the `tasks` array — throws `TaskNotFoundError` for dangling references
+  - Per-edge `qualityRetention` from the `DependencyEdge` objects
+  - Duplicate task IDs throw `DuplicateNodeError`
+  - Duplicate edges (same prerequisite→dependent pair) throw `DuplicateEdgeError`
+- [ ] `TaskGraph.fromJSON(data: TaskGraphSerialized): TaskGraph`:
+  - Validates input against `TaskGraphSerialized` schema (using TypeBox `Value.Check`)
+  - Uses `graph.import()` on the validated data
+  - Orphan nodes in JSON are preserved
+- [ ] `addTask(id: string, attributes: TaskGraphNodeAttributes): void`:
+  - Throws `DuplicateNodeError` if ID already exists
+  - Adds node to internal graphology instance
+- [ ] `addDependency(prerequisite: string, dependent: string, qualityRetention?: number): void`:
+  - Throws `TaskNotFoundError` if either endpoint doesn't exist
+  - Throws `DuplicateEdgeError` if edge already exists
+  - Uses `addEdgeWithKey` with deterministic key `${prerequisite}->${dependent}`
+  - Default `qualityRetention: 0.9` if not provided
+- [ ] `fromTasks`/`fromRecords` strip `null` → `undefined` for categorical fields during `TaskInput` → `TaskGraphNodeAttributes` transformation
+- [ ] `TaskInput` fields `tags`, `assignee`, `due`, `created`, `modified` are not stored on graph nodes (belong to caller)
+- [ ] Unit tests for each construction method: happy path, error cases, edge cases (empty arrays, cycles not rejected at construction time)
+- [ ] All construction methods use deterministic edge keys per ADR-006
+
+## References
+
+- docs/architecture/graph-model.md — construction paths, TaskInput→attributes transformation, error handling table
+- docs/architecture/api-surface.md — TaskGraph class, fromTasks/fromRecords/fromJSON/addTask/addDependency
+- docs/architecture/decisions/006-deterministic-edge-keys.md — deterministic edge keys
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion