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/frontmatter/file-io-and-serialize.md

@@ -0,0 +1,54 @@
+---
+id: frontmatter/file-io-and-serialize
+name: Implement parseTaskFile, parseTaskDirectory, and serializeFrontmatter
+status: pending
+depends_on:
+  - frontmatter/parsing
+  - schema/input-schemas
+scope: moderate
+risk: low
+impact: component
+level: implementation
+---
+
+## Description
+
+Implement the file I/O frontmatter functions and the serializer. These are convenience wrappers for common use cases and the reverse operation (TaskInput → markdown).
+
+Per [frontmatter.md](../../../docs/architecture/frontmatter.md):
+- `parseTaskFile`/`parseTaskDirectory` depend on `node:fs/promises` (Node.js only)
+- `parseFrontmatter` is runtime-agnostic
+- `serializeFrontmatter` uses `yaml.stringify()` for the data portion
+
+## Acceptance Criteria
+
+- [ ] `parseTaskFile(filePath: string): Promise<TaskInput>`:
+  - Reads file using `node:fs/promises.readFile`
+  - Delegates to `parseFrontmatter` for parsing and validation
+  - Throws underlying Node.js error for I/O failures (ENOENT, EACCES, etc.)
+- [ ] `parseTaskDirectory(dirPath: string): Promise<TaskInput[]>`:
+  - Recursive directory scanning via `node:fs/promises.readdir` with `{ recursive: true }` or manual recursion
+  - Filters for `.md` files only
+  - Silently skips files without valid `---`-delimited frontmatter (no error thrown, just omitted from results)
+  - Throws underlying Node.js error for I/O failures
+  - Uses `parseTaskFile` per file
+- [ ] `serializeFrontmatter(task: TaskInput, body?: string): string`:
+  - Constructs `---`-delimited markdown output
+  - Uses `yaml.stringify()` for the `TaskInput` data (excludes `id` from frontmatter? No — per Rust CLI convention, `id` comes from the filename, but in the schema it's part of `TaskInput`. Follow schema: include all `TaskInput` fields in the YAML.)
+  - Appends body content (default: empty string) after closing `---`
+  - Handles nullable fields correctly: `risk: null` → `risk: null` in YAML (explicit null), absent fields → omitted from YAML
+- [ ] File I/O functions documented as Node.js-only in JSDoc comments
+- [ ] Unit tests: parseTaskFile with temp file, parseTaskDirectory with temp dir (including non-.md files, missing frontmatter files), serializeFrontmatter round-trip parseFrontmatter(serializeFrontmatter(task)) ≈ task
+
+## References
+
+- docs/architecture/frontmatter.md — file I/O functions, splitter, serializer
+- docs/architecture/schemas.md — TaskInput definition for serialization
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion