fix(tier-check): add missing markdown table headers for repository health

[anubhav756]

Fixes a bug in the Markdown generator for the tier-check tool where the Repository Health section was missing a table header.

Motivation and Context

When running npx @modelcontextprotocol/conformance tier-check --output markdown, the resulting Markdown for the "Repository Health" section lacked a header row and separator row. According to the GFM specification, tables without headers are not parsed as HTML tables. This caused the output to render as raw, unformatted text when piped into GitHub Issues. This change adds the missing | Metric | Status | Details | header to ensure proper rendering.

How Has This Been Tested?

Tested locally by running the tier-check command with --output markdown against an SDK repository and verifying that the resulting output renders correctly as a 3-column table in a standard GitHub Issue markdown preview.

Breaking Changes

None. Users consuming the Markdown output will simply see it render correctly now.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

This is particularly useful for SDK maintainers who use GitHub Actions to pipe the output of the tier-check command directly into automated, nightly GitHub Issues to track their operational SLAs!