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/mutation.md

@@ -0,0 +1,38 @@
+---
+id: graph/mutation
+name: Implement TaskGraph mutation methods (remove, update, updateEdgeAttributes)
+status: pending
+depends_on:
+  - graph/taskgraph-class
+scope: narrow
+risk: low
+impact: component
+level: implementation
+---
+
+## Description
+
+Implement mutation methods in `src/graph/mutation.ts` and integrate on `TaskGraph`. These methods modify the graph in-place.
+
+## Acceptance Criteria
+
+- [ ] `removeTask(id: string): void` — No-op if node doesn't exist. Removes node and cascades edge removal (graphology handles this automatically).
+- [ ] `removeDependency(prerequisite: string, dependent: string): void` — No-op if edge doesn't exist. Uses deterministic edge key `${prerequisite}->${dependent}` to identify the edge.
+- [ ] `updateTask(id: string, attributes: Partial<TaskGraphNodeAttributes>): void` — Throws `TaskNotFoundError` if ID doesn't exist. Uses `mergeNodeAttributes` for shallow merge of provided attributes.
+- [ ] `updateEdgeAttributes(prerequisite: string, dependent: string, attrs: Partial<TaskGraphEdgeAttributes>): void` — Throws `TaskNotFoundError` (actually `TaskNotFoundError` for the edge itself, but per the spec, edge attributes need both endpoints to exist) if the edge doesn't exist. Uses `mergeEdgeAttributes` for shallow merge.
+- [ ] All mutations maintain the deterministic edge key format
+- [ ] Unit tests: remove nonexistent node/edge is no-op, update nonexistent throws, partial updates merge correctly
+
+## References
+
+- docs/architecture/api-surface.md — mutation methods
+- docs/architecture/errors-validation.md — mutation operation behavior table
+- docs/architecture/graph-model.md — edge attributes, mutation semantics
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion