feat: add Mermaid diagram export option

[xiapeli]

Summary

FlowMaster generates flow diagrams as Mermaid syntax inline, but there is no dedicated export functionality. Users should be able to export diagrams to standalone .mmd files or render them to SVG/PNG for documentation and presentations.

What needs to happen

1. Add an export instruction/command in the skill file that outputs:
   • .mmd file (raw Mermaid syntax, ready for Mermaid Live Editor)
   • .svg file (rendered diagram, if mmdc CLI from @mermaid-js/mermaid-cli is available)
2. Include suggested filename conventions (e.g., flow-<project-name>-<date>.mmd)
3. Add export examples to the documentation

Acceptance criteria

• Mermaid file can be opened in Mermaid Live Editor without errors
• SVG export works when mmdc is installed
• Graceful fallback message when mmdc is not available

Getting started

1. Review the current diagram output format in flowmaster.md
2. Test exporting sample output to .mmd files
3. Add export instructions to the skill file