slack: use slackify-markdown for proper mrkdwn formatting

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)

pyproject.toml

@@ -36,6 +36,7 @@ dependencies = [
     "python-socketio>=5.11.0",
     "msgpack>=1.0.8",
     "slack-sdk>=3.26.0",
+    "slackify-markdown>=0.2.0",
     "qq-botpy>=1.0.0",
     "python-socks[asyncio]>=2.4.0",
     "prompt-toolkit>=3.0.0",