Add Markdown output to dispatch info

[jongio]

dispatch info is useful in the terminal, but pasting the output into an issue, PR, or notes doc takes cleanup.

Add dispatch info --markdown <session> to print the same concise session summary as Markdown.

Acceptance criteria:

• dispatch info <id> --markdown prints a Markdown section with session fields and counts.
• --markdown works with --refs.
• --markdown cannot be combined with --json.
• Tests cover parsing and output.