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/setup/project-init.md

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+---
+id: setup/project-init
+name: Initialize project with package.json, tsconfig, and build tooling
+status: pending
+depends_on: []
+scope: moderate
+risk: low
+impact: project
+level: implementation
+---
+
+## Description
+
+Set up the TypeScript project from scratch. This is a greenfield project — the repo currently has only `AGENTS.md` and `docs/`. Initialize everything needed for a pure TypeScript ESM library: package.json, tsconfig.json, gitignore, and the src/ directory skeleton.
+
+Per [build-distribution.md](../../../docs/architecture/build-distribution.md):
+- Package name: `@alkdev/taskgraph`
+- ESM primary, CJS compat
+- Targets: Node 18+, Deno, Bun (pure JS, no native addons)
+- Build: `tsc` for declarations + bundler for distribution
+- Dependencies: `graphology`, `graphology-dag`, `graphology-metrics`, `graphology-components`, `graphology-operators`, `@alkdev/typebox`, `yaml`
+
+## Acceptance Criteria
+
+- [ ] `package.json` exists with name `@alkdev/taskgraph`, ESM primary (`"type": "module"`), CJS compat config
+- [ ] All production dependencies listed per [build-distribution.md](../../../docs/architecture/build-distribution.md) dependencies table
+- [ ] Dev dependencies include: `typescript`, `vitest` (or agreed test runner), `@types/node`
+- [ ] `tsconfig.json` configured for Node 18+ target, ESM module resolution, strict mode, declaration output
+- [ ] `.gitignore` covers `node_modules/`, `dist/`, `*.js.map`, `.env`
+- [ ] `src/` directory skeleton created per [build-distribution.md](../../../docs/architecture/build-distribution.md) project structure:
+  - `src/index.ts`
+  - `src/schema/index.ts`, `src/schema/enums.ts`, `src/schema/task.ts`, `src/schema/graph.ts`, `src/schema/results.ts`
+  - `src/graph/index.ts`, `src/graph/construction.ts`, `src/graph/queries.ts`, `src/graph/mutation.ts`
+  - `src/analysis/index.ts`, `src/analysis/critical-path.ts`, `src/analysis/bottleneck.ts`, `src/analysis/risk.ts`, `src/analysis/cost-benefit.ts`, `src/analysis/decompose.ts`, `src/analysis/defaults.ts`
+  - `src/frontmatter/index.ts`, `src/frontmatter/parse.ts`, `src/frontmatter/serialize.ts`
+  - `src/error/index.ts`
+- [ ] `test/` directory created with placeholder test files per build-distribution spec
+- [ ] `npm install` succeeds without errors
+- [ ] `npx tsc --noEmit` succeeds (empty source files, but config is valid)
+
+## References
+
+- docs/architecture/build-distribution.md — project structure, dependencies, targets
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion

tasks/implementation/setup/test-infrastructure.md

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+---
+id: setup/test-infrastructure
+name: Configure test runner and shared test fixtures
+status: pending
+depends_on:
+  - setup/project-init
+scope: narrow
+risk: low
+impact: project
+level: implementation
+---
+
+## Description
+
+Set up the test infrastructure: configure Vitest (or chosen runner), create shared test fixtures and helpers for graph construction that all downstream test files will use. This avoids every test file building graphs from scratch.
+
+## Acceptance Criteria
+
+- [ ] Test runner configured in `package.json` scripts (`"test"`, `"test:watch"`, `"test:coverage"`)
+- [ ] Vitest config (or equivalent) exists with ESM support and TypeScript path resolution
+- [ ] Shared test fixture file created (e.g., `test/fixtures/graphs.ts`) with:
+  - A simple linear chain graph (3-4 tasks, A→B→C→D)
+  - A diamond dependency graph (A→B, A→C, B→D, C→D)
+  - A graph with mixed categorical fields (some assessed, some null)
+  - A graph with cycles for testing cycle detection
+  - A larger graph (20+ nodes) for performance/bottleneck testing
+- [ ] Helper function to create a `TaskGraph` from `TaskInput[]` for one-liner test setup
+- [ ] Test runner executes successfully against placeholder test files
+- [ ] CI-compatible output format (no watch mode in default script)
+
+## References
+
+- docs/architecture/build-distribution.md — test directory structure
+- docs/architecture/graph-model.md — graph construction examples for fixtures
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion