Add Markdown output to dispatch compare #432

dispatch compare renders text or JSON. Add a --markdown flag so the comparison can be dropped straight into a PR description, issue, or doc.

Acceptance criteria:

• dispatch compare --markdown prints the comparison as Markdown (headings and/or tables).
• --json and --markdown cannot be combined and return a clear error.
• README and man page document the flag.
• Tests cover the Markdown rendering and the mutually-exclusive flag error.