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/splitter.md

@@ -0,0 +1,45 @@
+---
+id: frontmatter/splitter
+name: Implement frontmatter delimiter splitter (~40 lines)
+status: pending
+depends_on:
+  - setup/project-init
+scope: single
+risk: trivial
+impact: component
+level: implementation
+---
+
+## Description
+
+Implement the self-contained `---` delimited frontmatter splitter in `src/frontmatter/parse.ts`. This is a ~40 line function that extracts the YAML data string and markdown content body from a markdown string. No gray-matter dependency.
+
+Per [frontmatter.md](../../../docs/architecture/frontmatter.md), the splitter:
+1. Checks for opening `---` delimiter (not `----`)
+2. Finds closing `\n---` delimiter
+3. Extracts YAML data string and markdown content body
+4. Returns `{ data: string, content: string }` or `null` if no valid frontmatter
+
+## Acceptance Criteria
+
+- [ ] `splitFrontmatter(markdown: string): { data: string; content: string } | null`
+- [ ] Opening `---` must be at the start of the file (or after optional BOM/whitespace on first line)
+- [ ] `----` (4+ dashes) is NOT a valid delimiter — only exact `---`
+- [ ] Closing delimiter requires `\n---` (newline before dashes)
+- [ ] Returns `null` if no valid frontmatter found
+- [ ] Returns `{ data: "", content: "" }` if frontmatter is present but empty (e.g., `---\n---`)
+- [ ] Content body starts after the closing `---` + newline
+- [ ] Handles edge cases: no closing delimiter (returns null), file with only `---\n---`, file with no `---` at all
+- [ ] Unit tests: standard frontmatter, no frontmatter, empty frontmatter, multi-line content, dashes in content body (shouldn't be treated as delimiters), 4+ dashes ignored
+
+## References
+
+- docs/architecture/frontmatter.md — splitter design, supply chain decision
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion