feat(frontmatter/splitter): implement splitFrontmatter function with tests

tasks/implementation/frontmatter/splitter.md

@@ -1,7 +1,7 @@
 ---
 id: frontmatter/splitter
 name: Implement frontmatter delimiter splitter (~40 lines)
-status: pending
+status: completed
 depends_on:
   - setup/project-init
 scope: single
@@ -22,15 +22,15 @@ Per [frontmatter.md](../../../docs/architecture/frontmatter.md), the splitter:
 
 ## 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
+- [x] `splitFrontmatter(markdown: string): { data: string; content: string } | null`
+- [x] Opening `---` must be at the start of the file (or after optional BOM/whitespace on first line)
+- [x] `----` (4+ dashes) is NOT a valid delimiter — only exact `---`
+- [x] Closing delimiter requires `\n---` (newline before dashes)
+- [x] Returns `null` if no valid frontmatter found
+- [x] Returns `{ data: "", content: "" }` if frontmatter is present but empty (e.g., `---\n---`)
+- [x] Content body starts after the closing `---` + newline
+- [x] Handles edge cases: no closing delimiter (returns null), file with only `---\n---`, file with no `---` at all
+- [x] 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
 
@@ -38,8 +38,13 @@ Per [frontmatter.md](../../../docs/architecture/frontmatter.md), the splitter:
 
 ## Notes
 
-> To be filled by implementation agent
+Self-contained `splitFrontmatter` function implemented with no external dependencies. Uses regex for opening delimiter match and manual scan for closing delimiter to enforce exact 3-dash rule. Handles BOM stripping and empty frontmatter.
 
 ## Summary
 
-> To be filled on completion
+Implemented the `splitFrontmatter` function in `src/frontmatter/parse.ts` per architecture spec.
+- Modified: `src/frontmatter/parse.ts` (added `splitFrontmatter` function, ~65 lines including JSDoc)
+- Modified: `src/frontmatter/index.ts` (exported `splitFrontmatter`)
+- Modified: `test/frontmatter.test.ts` (18 comprehensive tests)
+- Tests: 18 splitFrontmatter tests + 4 existing placeholder tests, all passing (22 total)
+- TypeScript check: passing