fix: Slack message formatting is ugly — markdown not rendering properly

[vybe]

Summary

Agent responses displayed in Slack have poor formatting despite the SLACK-RICHTEXT feature (#223) being implemented. Messages appear ugly/unreadable — likely markdown syntax showing as raw text or Block Kit conversion not handling all cases.

Current Behavior

Agent responses in Slack display with poor formatting. Needs investigation to identify specific issues (raw markdown showing through, missing code block formatting, broken lists, etc.).

Expected Behavior

Agent responses should render cleanly in Slack using Block Kit rich text formatting, with proper handling of:

• Code blocks (inline and fenced)
• Bullet/numbered lists
• Bold/italic text
• Headers (converted to bold since Slack has no headers in messages)
• Links
• Line breaks and paragraph spacing

Investigation Steps

1. Send a test message with mixed markdown (code, lists, bold, links)
2. Compare raw agent response with what Slack displays
3. Identify which markdown elements are not converting correctly
4. Check slack_adapter.py and any markdown-to-mrkdwn conversion logic

Acceptance Criteria

• Code blocks render as Slack code blocks
• Lists render as formatted lists
• Bold/italic text renders correctly
• Messages are readable and well-structured in Slack

[pavshulin]

/claim

[github-actions]

Claimed by @pavshulin. Label status-in-progress added.

Next steps:

1. Create branch: feature/293-<short-slug>
2. Implement the feature
3. Open a PR with Fixes #293