Feature request: render mermaid code fences in markdown messages

Summary

CodexMonitor currently does not render fenced mermaid code blocks in message markdown. Rendering Mermaid diagrams would allow users to discuss architecture and sequence diagrams directly with CodexMonitor

Current behavior (Codex Monitor)

Mermaid blocks are not rendered as diagrams in CodexMonitor.

Expected behavior (Codex)

Fenced mermaid blocks should render as Mermaid diagrams like Codex.

Suggested scope

A minimal implementation could:

• detect fenced code blocks with language mermaid
• render them as Mermaid diagrams in message markdown
• keep existing rendering unchanged for other code block languages
• fall back to the normal code block if Mermaid rendering fails