Merge graph/construction: fromTasks, fromRecords, fromJSON, addTask, addDependency

# Conflicts:
#	src/graph/construction.ts

tasks/implementation/graph/construction.md

@@ -1,7 +1,7 @@
 ---
 id: graph/construction
 name: Implement TaskGraph construction methods (fromTasks, fromRecords, fromJSON, addTask, addDependency)
-status: pending
+status: completed
 depends_on:
   - graph/taskgraph-class
 scope: broad
@@ -18,34 +18,34 @@ Per [graph-model.md](../../../docs/architecture/graph-model.md), the preferred i
 
 ## Acceptance Criteria
 
-- [ ] `TaskGraph.fromTasks(tasks: TaskInput[]): TaskGraph`:
+- [x] `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`:
+- [x] `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`:
+- [x] `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`:
+- [x] `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`:
+- [x] `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
+- [x] `fromTasks`/`fromRecords` strip `null` → `undefined` for categorical fields during `TaskInput` → `TaskGraphNodeAttributes` transformation
+- [x] `TaskInput` fields `tags`, `assignee`, `due`, `created`, `modified` are not stored on graph nodes (belong to caller)
+- [x] Unit tests for each construction method: happy path, error cases, edge cases (empty arrays, cycles not rejected at construction time)
+- [x] All construction methods use deterministic edge keys per ADR-006
 
 ## References
 
@@ -55,8 +55,17 @@ Per [graph-model.md](../../../docs/architecture/graph-model.md), the preferred i
 
 ## Notes
 
-> To be filled by implementation agent
+Implementation approach:
+- `fromTasks` and `fromRecords` use the bulk `graph.import()` approach per architecture recommendation (build serialized blob, import in one call).
+- `fromJSON` adds TypeBox `Value.Check()` validation before importing, throwing `InvalidInputError` for schema violations.
+- `fromTasks` creates orphan nodes for dangling dependsOn references with default attributes `{ name: <dep-id> }`.
+- `fromRecords` is strict — any dangling edge reference throws `TaskNotFoundError`.
+- `addTask` and `addDependency` are thin wrappers with validation on the underlying graphology instance.
+- The `taskInputToNodeAttrs` helper strips null→undefined for categorical fields and drops non-graph fields (tags, assignee, due, created, modified).
 
 ## Summary
 
-> To be filled on completion
+Implemented all five construction methods on the TaskGraph class.
+- Modified: `src/graph/construction.ts` (full implementation of fromTasks, fromRecords, fromJSON with validation, addTask, addDependency; taskInputToNodeAttrs helper)
+- Modified: `test/graph.test.ts` (added 47 new tests for construction methods, preserved 42 existing tests)
+- Tests: 304 total (all passing), lint clean