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/parsing.md

@@ -0,0 +1,51 @@
+---
+id: frontmatter/parsing
+name: Implement parseFrontmatter with YAML parsing and TypeBox validation
+status: pending
+depends_on:
+  - frontmatter/splitter
+  - schema/input-schemas
+  - error/error-hierarchy
+scope: narrow
+risk: low
+impact: component
+level: implementation
+---
+
+## Description
+
+Implement `parseFrontmatter(markdown: string): TaskInput` in `src/frontmatter/parse.ts`. This combines the splitter with `yaml.parse()` and TypeBox validation. Invalid frontmatter throws `InvalidInputError` with field-level details.
+
+Per [frontmatter.md](../../../docs/architecture/frontmatter.md), the function uses:
+- The custom splitter for `---` extraction
+- `yaml.parse()` (from `yaml` package, zero dependencies) for YAML↔JS conversion
+- TypeBox `Value.Check()` + `Value.Errors()` for structured field-level validation
+- `Value.Clean()` to strip unknown properties from untrusted input
+
+## Acceptance Criteria
+
+- [ ] `parseFrontmatter(markdown: string): TaskInput`:
+  - Calls splitter to extract YAML string
+  - Throws `InvalidInputError` if no valid frontmatter found (not `null` return — the caller expects TaskInput)
+  - Calls `yaml.parse(yamlString)` for YAML 1.2 parsing
+  - Runs `Value.Clean(TaskInput, parsed)` to strip unknown properties
+  - Runs `Value.Check(TaskInput, cleaned)` — if fails, runs `Value.Errors()` and throws `InvalidInputError` with structured field/path/message/value details
+  - Returns validated `TaskInput`
+- [ ] `InvalidInputError` is populated with field-level details from `Value.Errors()` output
+- [ ] YAML 1.2 used exclusively (the `yaml` package default) — no YAML 1.1 type coercion
+- [ ] Handles YAML `null` values (e.g., `risk:` with no value) correctly — becomes `null` in the TaskInput (distinction from absent field)
+- [ ] Unit tests: valid frontmatter, missing required fields, invalid enum values, unknown fields stripped, null categorical values preserved
+
+## References
+
+- docs/architecture/frontmatter.md — parseFrontmatter, yaml package, no gray-matter
+- docs/architecture/schemas.md — TaskInput schema, Nullable helper
+- docs/architecture/errors-validation.md — InvalidInputError
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion