Add MarkdownToSlackMrkdwn() converter that transforms standard Markdown to Slack's mrkdwn format before sending messages: - **bold** → *bold* - ***bold italic*** → *_bold italic_* - ~~strike~~ → ~strike~ - [text](url) → <url|text> - # Heading → *Heading* - Code blocks and inline code are preserved untouched. Applied in both Reply() and Send() methods of the Slack platform. Closes #667 Co-authored-by: Claude <noreply@anthropic.com>