Enable rich-text messages in matrix channel

[dim]

📝 Description

🗣️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated (100% AI, 0% Human)
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨‍💻 Mostly Human-written (Human lead, AI assisted or none)

🔗 Related Issue

📚 Technical Context (Skip for Docs)

• Reference URL: n/a
• Reasoning: Messages are being sent as markdown but not picked up in common Matrix client, e.g. Element or FluffyChat

🧪 Test Environment

• Hardware: Raspberry Pi 5
• OS: Debian 13
• Model/Provider: Gemini
• Channels: Matrix

📸 Evidence (Optional)

Details

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.

[CLAassistant]

All committers have signed the CLA.

[yinwm]

Review Summary

This PR adds rich-text message support for the Matrix channel by converting Markdown to HTML format. The implementation is clean and well-structured.

✅ What's Good

1. Clean code reuse: The messageContent() method is properly shared between Send() and EditMessage(), eliminating code duplication.

2. Correct Matrix spec compliance: The implementation correctly keeps the original Markdown in Body as fallback while providing HTML in FormattedBody:

   mc := &event.MessageEventContent{MsgType: event.MsgText, Body: text}  // fallback
   mc.FormattedBody = markdownToHTML(text)  // rich text

3. Sensible defaults: Richtext is enabled by default (when MessageFormat is empty or "richtext"), with explicit "plain" option for users who prefer raw Markdown.

4. Good test coverage: Both TestMarkdownToHTML and TestMessageContent cover the key scenarios including richtext, plain, and default configurations.

5. Documentation: README is updated with clear explanation of the new message_format option.

🔒 Security

The gomarkdown/markdown library doesn't parse raw HTML by default, and Matrix clients are responsible for HTML sanitization per the spec. The generated tags (<strong>, <em>, <code>, <pre>, <a>, headers, lists, etc.) are all within Matrix's allowed HTML subset.

LGTM! 🚀

[yinwm]

thanks for the pr