Add Markdown output to dispatch tags

[jongio]

dispatch tags already supports text and JSON output. Markdown would make tag counts easier to paste into issues, PRs, and status reports.

Acceptance criteria:

• dispatch tags --markdown prints a summary table and a tag count table.
• Empty tag sets print a short Markdown empty-state message.
• --markdown cannot be combined with --json.
• Tests cover Markdown output and the flag conflict.