Agents output standard Markdown (Claude Code default) but Slack uses
its own mrkdwn format. Without conversion:
**bold** shows as literal **bold**
### heading shows as literal ###
[text](url) shows as raw markdown link
Converter handles:
**bold** → *bold* (Slack bold is single asterisk)
### heading → *heading* (bold text, no headings in Slack)
[text](url) → <url|text> (Slack link format)
--- → ——— (visual separator)
`code` and ```blocks``` pass through unchanged
6 new tests: bold, heading, link, hr, code block, mixed.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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