Replace the regex-based Markdown-to-Slack converter with the
slackify-markdown library, which uses a proper Markdown parser
(markdown-it-py, already a dependency) to correctly handle headings,
bold/italic, code blocks, links, bullet lists, and strikethrough.
The regex approach didn't handle headings (###), bullet lists (* ),
or code block protection, causing raw Markdown to leak into Slack
messages.
Net -40 lines.
Assisted-by: Claude 4.6 Opus (Anthropic)
