Problem

The Slack CLI tool accepts messages but the Slack API uses Mrkdwn format, which differs from standard Markdown. Users need to manually convert their Markdown-formatted messages, which is inconvenient for coding agents and automation tools.

Solution

This PR implements automatic Markdown to Mrkdwn conversion, allowing users to write messages in standard Markdown format while ensuring compatibility with Slack's formatting requirements.

Key Changes

Automatic Format Conversion (markdown.go)

• Converts standard Markdown syntax to Slack's Mrkdwn format transparently
• Uses regex patterns with placeholder protection to handle nested formatting correctly
• Supports all common Markdown elements:
  • Bold: **text** or __text__ → *text*
  • Italic: *text* → _text*
  • Strikethrough: ~~text~~ → ~text~
  • Links: [text](url) → <url|text>
  • Code blocks with language specifiers (including c++, c#, objective-c)
  • Unordered lists: * item or - item → • item

Example Conversion

Input:  **Status**: Build ~~failed~~ *succeeded*! Check [logs](https://example.com/logs)
Output: *Status*: Build ~failed~ _succeeded_! Check <https://example.com/logs|logs>

Testing (markdown_test.go)

• 15 comprehensive test cases covering all conversion scenarios
• Edge cases including complex language specifiers and mixed formatting
• All tests passing

Documentation Updates (README.md)

• Updated agent prompt to mention automatic Markdown conversion

Benefits

• No breaking changes: Existing usage continues to work
• Zero dependencies: Uses only Go standard library
• Transparent: Conversion happens automatically
• Well-tested: Comprehensive test coverage
• Secure: CodeQL scan found no vulnerabilities
• Agent-friendly: Makes it easier for coding agents to send formatted Slack messages

Users can now send messages using familiar Markdown syntax without worrying about Slack's formatting differences.

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