Handle empty frontmatter in local helpers

[fa1k3]

Summary

• recognize empty YAML frontmatter blocks (---\n---\n) in the local frontmatter helpers
• anchor the matcher to the start of the document (no /m flag) so --- lines in the body (thematic breaks) are never mistaken for a frontmatter block
• avoid duplicating ---\n--- when adding frontmatter tags to a note with an empty block
• LF/CRLF empty-block coverage plus a regression test for body-only --- lines

Why

The composed tag/frontmatter helpers read and rewrite raw note content for operations that have no single Local REST API equivalent. The matcher missed valid empty frontmatter blocks.

A multiline-anchored fix would over-match: with /m, the opening ^--- can match a --- line mid-body, and because splice() slices via m[0].length (assuming the match is at index 0), a note with no frontmatter but --- lines in the body would be silently corrupted. The matcher is therefore anchored to string start with an optional content group — this handles the empty block without the false positive.

Tests

• bun test tests/services/frontmatter-ops.test.ts
• bun run test (full suite green)
• bun run test:types

[fa1k3]

Updated: rebased onto main. The matcher is now anchored to the start of the document rather than using /m — a multiline anchor lets the opening ^--- match a --- line mid-body, and since splice() slices via m[0].length (assuming a match at index 0), that silently corrupts notes that have no frontmatter but contain --- lines. Added a regression test for that case alongside the empty-block coverage. Ready for review.