The slackify_markdown library misses several patterns that LLMs commonly
produce, causing raw Markdown symbols (**bold**, ##headers) to appear
in Slack messages.
Add _fixup_mrkdwn() post-processor that:
- Converts leftover **bold** patterns (e.g. **Status:**OK where closing
** is adjacent to non-space chars)
- Fixes & over-escaping in bare URLs
- Protects code blocks from false-positive fixups
Co-authored-by: Cursor <cursoragent@cursor.com>
