Issue Description

Validate-MarkdownFrontmatter.ps1 contains a Get-ChangedMarkdownFileGroup function that duplicates functionality already available in LintingHelpers.psm1 (Get-ChangedFilesFromGit). The script should use the shared module function instead of reimplementing git diff parsing.

Additional Context

Current state:

• scripts/linting/Validate-MarkdownFrontmatter.ps1 has Get-ChangedMarkdownFileGroup (~40 lines)
• scripts/linting/Modules/LintingHelpers.psm1 exports Get-ChangedFilesFromGit with similar logic

Recommended approach:

1. Import LintingHelpers.psm1 at the top of Validate-MarkdownFrontmatter.ps1
2. Replace Get-ChangedMarkdownFileGroup calls with Get-ChangedFilesFromGit filtered by .md extension
3. Remove the duplicate function

Testing:

• Run npm run pester for existing tests
• Test with npm run lint:frontmatter on changed markdown files
• Verify -ChangesOnly parameter still works correctly