The frontmatter parser was using a simple line-by-line regex that only
captured single-line key-value pairs. This meant multi-line metadata
blocks (as used by bundled hooks) were not parsed correctly.
Changes:
- Add extractMultiLineValue() to handle indented continuation lines
- Use JSON5 instead of JSON.parse() to support trailing commas
- Add comprehensive test coverage for frontmatter parsing
Fixes#1113
