Implements #137: markdown parsing and section extraction for help system.
- Add Topic and Section types for help content structure
- Add Frontmatter type for YAML metadata parsing
- Add ParseTopic() to parse markdown files into Topic structs
- Add ExtractFrontmatter() to extract YAML frontmatter
- Add ExtractSections() to extract headings and content
- Add GenerateID() to create URL-safe anchor IDs
- Add comprehensive tests following _Good/_Bad naming convention
This is the foundation for the display-agnostic help system (#133).
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
