Epic: #1978

Slack channel sends raw markdown which doesn't render correctly in Slack (uses mrkdwn format). Implement conversion similar to Telegram's markdown_to_telegram().

Acceptance Criteria:

• Slack send() converts markdown to mrkdwn
• Bold, italic, code, links, headers converted
• Unit tests for conversion

Complexity: Small | Effort: 2h
Ref: CHAN-06