feat(schema/graph-schemas): define TaskGraphNodeAttributes, TaskGraphEdgeAttributes, and SerializedGraph generic

Implements graph attribute schemas and the SerializedGraph generic factory
parameterized with <N, E, G> following the graphology JSON format.

- TaskGraphNodeAttributes: name + optional categorical enums (scope, risk,
  impact, level, priority, status) — analysis-relevant metadata only
- TaskGraphNodeAttributesUpdate: Type.Partial(TaskGraphNodeAttributes)
- TaskGraphEdgeAttributes: optional qualityRetention number
- SerializedGraph<N, E, G>: generic factory for graphology JSON format
- TaskGraphSerialized: concrete instantiation with empty graph attributes
- No schema version field per spec

35 new tests covering validation, rejection, and compile-time type safety.

tasks/implementation/schema/graph-schemas.md

@@ -1,7 +1,7 @@
 ---
 id: schema/graph-schemas
 name: Define TaskGraphNodeAttributes, TaskGraphEdgeAttributes, and SerializedGraph
-status: pending
+status: completed
 depends_on:
   - schema/enums
 scope: narrow
@@ -16,17 +16,17 @@ Define graph attribute schemas and the serialized graph generic in `src/schema/g
 
 ## Acceptance Criteria
 
-- [ ] `src/schema/graph.ts` exports:
-  - `TaskGraphNodeAttributes` schema: `name: Type.String()`, optional categorical enums (scope, risk, impact, level, priority, status) — **not** nullable on the graph (absent = not stored)
-  - `type TaskGraphNodeAttributes` derived
-  - `TaskGraphNodeAttributesUpdate = Type.Partial(TaskGraphNodeAttributes)` and type alias
-  - `TaskGraphEdgeAttributes` schema: `qualityRetention: Type.Optional(Type.Number())`
-  - `type TaskGraphEdgeAttributes` derived
-  - `SerializedGraph` generic factory parameterized with `<N extends TSchema, E extends TSchema, G extends TSchema>`
-  - `TaskGraphSerialized = SerializedGraph(TaskGraphNodeAttributes, TaskGraphEdgeAttributes, Type.Object({}))` and type alias
-- [ ] `SerializedGraph` generic follows graphology JSON format: `attributes`, `options: { type: "directed", multi: false, allowSelfLoops: false }`, `nodes: [{ key, attributes }]`, `edges: [{ key, source, target, attributes }]`
-- [ ] No schema version field on `TaskGraphSerialized` per spec
-- [ ] Re-exported from `src/schema/index.ts`
+- [x] `src/schema/graph.ts` exports:
+  - [x] `TaskGraphNodeAttributes` schema: `name: Type.String()`, optional categorical enums (scope, risk, impact, level, priority, status) — **not** nullable on the graph (absent = not stored)
+  - [x] `type TaskGraphNodeAttributes` derived
+  - [x] `TaskGraphNodeAttributesUpdate = Type.Partial(TaskGraphNodeAttributes)` and type alias
+  - [x] `TaskGraphEdgeAttributes` schema: `qualityRetention: Type.Optional(Type.Number())`
+  - [x] `type TaskGraphEdgeAttributes` derived
+  - [x] `SerializedGraph` generic factory parameterized with `<N extends TSchema, E extends TSchema, G extends TSchema>`
+  - [x] `TaskGraphSerialized = SerializedGraph(TaskGraphNodeAttributes, TaskGraphEdgeAttributes, Type.Object({}))` and type alias
+- [x] `SerializedGraph` generic follows graphology JSON format: `attributes`, `options: { type: "directed", multi: false, allowSelfLoops: false }`, `nodes: [{ key, attributes }]`, `edges: [{ key, source, target, attributes }]`
+- [x] No schema version field on `TaskGraphSerialized` per spec
+- [x] Re-exported from `src/schema/index.ts`
 
 ## References
 
@@ -35,8 +35,11 @@ Define graph attribute schemas and the serialized graph generic in `src/schema/g
 
 ## Notes
 
-> To be filled by implementation agent
+Follows the architecture spec in docs/architecture/schemas.md and the generic schema factory pattern from docs/research/typebox-patterns.md section 6. The `SerializedGraph` generic factory uses the recommended graphology JSON format with `Type.Literal("directed")`, `Type.Literal(false)`, and `Type.Literal(false)` for the options. No `default` on `qualityRetention` in the graph schema (unlike `DependencyEdge` which has `Type.Number({ default: 0.9 })`) — the graph schema keeps it simple with `Type.Optional(Type.Number())`.
 
 ## Summary
 
-> To be filled on completion
+Implemented graph attribute schemas and serialized graph generic factory per spec.
+- Created: `src/schema/graph.ts` (TaskGraphNodeAttributes, TaskGraphNodeAttributesUpdate, TaskGraphEdgeAttributes, SerializedGraph generic, TaskGraphSerialized, and all type aliases)
+- Modified: `test/schema.test.ts` (35 new tests for graph schemas: TaskGraphNodeAttributes, TaskGraphNodeAttributesUpdate, TaskGraphEdgeAttributes, SerializedGraph, and compile-time type verification)
+- Tests: 121 total, all passing; `tsc --noEmit` clean