feat: add Markdown report output

[gokul-debugger]

Summary

• add opt-in GitHub-flavored Markdown rendering for inspection reports
• support writing JSON or Markdown output to a file
• reject incompatible output options with clear CLI errors
• preserve the existing terminal report as the default

Testing

• python -m pytest (47 passed)
• ruff check .
• ruff format --check .
• mypy src tests
• git diff --check

Closes #1