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/subgraph-and-validation.md

@@ -0,0 +1,54 @@
+---
+id: graph/subgraph-and-validation
+name: Implement TaskGraph subgraph and validation methods
+status: pending
+depends_on:
+  - graph/taskgraph-class
+  - graph/queries
+  - schema/input-schemas
+  - schema/graph-schemas
+scope: narrow
+risk: low
+impact: component
+level: implementation
+---
+
+## Description
+
+Implement the `subgraph()` method and the three validation methods (`validateSchema`, `validateGraph`, `validate`) on `TaskGraph`.
+
+Per [ADR-007](../../../docs/architecture/decisions/007-subgraph-internal-only.md), `subgraph` returns only edges where both endpoints are in the filtered set.
+
+Per [errors-validation.md](../../../docs/architecture/errors-validation.md), validation methods collect issues and return arrays — never throw.
+
+## Acceptance Criteria
+
+- [ ] `subgraph(filter: (taskId: string, attrs: TaskGraphNodeAttributes) => boolean): TaskGraph`:
+  - Uses `graphology-operators.subgraph` to extract matching nodes
+  - Returns only edges where both endpoints are in the filtered set (internal-only) per ADR-007
+  - Returns a new `TaskGraph` instance (not mutating the original)
+- [ ] `validateSchema(): ValidationError[]`:
+  - Uses TypeBox `Value.Check()` and `Value.Errors()` on each node's attributes
+  - Returns structured `ValidationError[]` with `type: "schema"`, `taskId`, `field`, `message`, `value`
+- [ ] `validateGraph(): GraphValidationError[]`:
+  - Runs `findCycles()` and checks for dangling dependency references
+  - Returns structured `GraphValidationError[]` with `type: "graph"`, `category`, `message`, optional `details`
+  - Cycle category: `"cycle"` with cycle paths in `details`
+  - Dangling reference category: `"dangling-reference"` with the referencing task ID
+- [ ] `validate(): ValidationError[]` — runs both `validateSchema()` and `validateGraph()`, returns combined array
+- [ ] `ValidationError` and `GraphValidationError` interfaces defined (may be in error module or co-located)
+- [ ] Unit tests: subgraph filtering, subgraph excludes external edges, validateSchema catches invalid enums, validateGraph catches cycles and dangling refs
+
+## References
+
+- docs/architecture/api-surface.md — validation API, subgraph
+- docs/architecture/errors-validation.md — validation levels, return types
+- docs/architecture/decisions/007-subgraph-internal-only.md — subgraph semantics
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion