docs: create example output gallery in README

[xiapeli]

Summary

The README explains what FlowMaster does but does not show real example outputs. A visual gallery of sample diagrams for different project types would help users understand FlowMaster'''s capabilities at a glance.

What needs to happen

1. Generate example FlowMaster diagrams for 3-5 common project types:
   • Next.js App Router project
   • SvelteKit project
   • Astro project
   • Express.js API
   • A simple multi-page app
2. Add a "## Examples" or "## Gallery" section to README.md
3. Include both the rendered Mermaid diagram (using GitHub'''s native Mermaid rendering in ```mermaid blocks) and a brief description of what each diagram shows
4. Show both Analyze and Studio mode outputs if they differ

Acceptance criteria

• At least 3 example diagrams visible directly in the README on GitHub
• Each example includes: project type, brief description, rendered diagram
• Diagrams render correctly in GitHub'''s Mermaid preview
• Examples demonstrate different node types and layout patterns

Getting started

1. Run FlowMaster against a small sample project (or create a minimal one)
2. Capture the Mermaid output
3. Add it to README.md inside a ```mermaid code block
4. Verify it renders on GitHub