Issue Description

Add FooterExcludePaths parameter to exclude specific files from footer validation while retaining other frontmatter checks.

CHANGELOG.md is auto-generated by release-please and cannot maintain a footer. Currently, the only options are:

• Skip ALL footer validation globally (SkipFooterValidation)
• Exclude files from ALL validation (ExcludePaths)

Neither option allows CHANGELOG.md to retain frontmatter validation while skipping footer checks.

The solution adds a FooterExcludePaths parameter that accepts glob patterns for files to exclude from footer validation only, with a default of CHANGELOG.md.

Additional Context

Files to be changed:

• scripts/linting/Modules/FrontmatterValidation.psm1
• scripts/linting/Validate-MarkdownFrontmatter.ps1
• .github/workflows/frontmatter-validation.yml
• scripts/tests/linting/FrontmatterValidation.Tests.ps1
• scripts/tests/linting/Validate-MarkdownFrontmatter.Tests.ps1