feat: HTML and Markdown test report generation

[thomhurst]

Description

GitHub and JUnit reporters exist, but there's no HTML or Markdown report generation for email distribution, wiki publishing, or dashboard embedding.

Current State

• JUnit XML output
• GitHub Workflow summary
• JSON output
• No pretty HTML reports
• No markdown for documentation

Proposed Approach

• Create IReportGenerator interface with implementations: HtmlReportGenerator, MarkdownReportGenerator
• Add --report-format option: html, markdown
• Generate summary cards with pass rate, duration metrics, failed test details
• Create ReportTemplate system for customization