fix(frontmatter): recognise empty frontmatter blocks

[MFA-G]

Summary

splitFrontmatter failed to recognise an empty frontmatter block — a file that opens with --- and closes with --- on the next line, with no lines in between:

---
---
Body content

The existing regex required at least one captured line between the delimiters (([\s\S]*?)\r?\n---), so an empty block fell through to the no-frontmatter branch. Two consequences:

• hasFrontmatter was reported as false and the literal ---\n--- markers leaked into body.
• For subagent markdown files, extractRawFrontmatter then threw No YAML frontmatter found in file (see source/subagents/markdown-parser.ts), even though the file does have a (valid, empty) frontmatter block.

Fix

Make the inner frontmatter capture group optional so an empty block is matched, returning an empty frontmatter string and a clean body:

-const frontmatterRegex = /^---\s*\r?\n([\s\S]*?)\r?\n---\s*\r?\n?([\s\S]*)$/;
+const frontmatterRegex = /^---\s*\r?\n(?:([\s\S]*?)\r?\n)?---\s*\r?\n?([\s\S]*)$/;

I verified against a matrix of inputs that the only behavioural change is the previously-unhandled empty-block case — every other input (standard, multi-line, CRLF, no trailing newline, embedded --- in the body, lone delimiter, no frontmatter) splits identically to before.

Tests

Added source/utils/frontmatter.spec.ts (the module had no spec). It covers splitFrontmatter across all the cases above — including a regression test for the empty block — plus parseYamlObject (valid object, nested/array values, blank input, scalar, top-level array, invalid YAML).

Validation

• pnpm exec ava source/utils/frontmatter.spec.ts — 15 passed
• pnpm run test:ava — full suite, 5775 passed
• pnpm run test:types — clean
• pnpm exec biome lint/check (changed files) — clean
• pnpm run test:knip — clean

[Copilot]

Pull request overview

Fixes splitFrontmatter so it correctly recognizes empty YAML frontmatter blocks (---\n---\n...) at the start of markdown files, preventing delimiter leakage into the body and ensuring hasFrontmatter is reported correctly.

Changes:

• Updated the frontmatter regex to allow an empty frontmatter section between delimiters.
• Adjusted splitFrontmatter extraction to return frontmatter: '' when the optional capture group is absent.
• Added a new AVA spec file covering splitFrontmatter and parseYamlObject across a range of inputs, including the empty-frontmatter regression.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
source/utils/frontmatter.ts	Makes the frontmatter capture optional so ---\n---\n is treated as valid frontmatter with an empty frontmatter string.
source/utils/frontmatter.spec.ts	Adds coverage for empty/blank-line frontmatter handling and parseYamlObject behaviors (valid mapping, blank, scalar/array, invalid YAML).

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