Add comprehensive code walkthrough presentation for developers

[Copilot]

This PR adds a detailed code walkthrough presentation that provides an educational tour of the entire Marptalk codebase. The presentation is designed for developers who are moderately familiar with JavaScript but new to this specific project.

What's Included

34-slide comprehensive presentation (code-walkthrough.md) covering:

• Project Architecture: Overview of the modular design and technology stack
• Main Entry Point: Deep dive into generate.js with CLI setup using Commander.js
• Three-Stage Pipeline: Detailed explanation of each processing stage:
  • Stage A: Note extraction with extract-notes.js
  • Stage B: Audio generation with generate-audio.js
  • Stage C: HTML enhancement with generate-html.js
• Client-Side Automation: Complete walkthrough of slide-automation.js browser JavaScript
• Code Quality: Design principles and best practices demonstrated throughout
• Learning Opportunities: Key takeaways for JavaScript and Node.js developers

Educational Features

Each slide includes conversational speaker notes that:

• Explain complex concepts in accessible language
• Highlight modern JavaScript patterns (async/await, IIFE, Promise.all)
• Demonstrate Node.js best practices for file operations and CLI development
• Show real-world examples of browser JavaScript for DOM manipulation
• Connect code details to broader software engineering principles

Code Examples

The presentation includes carefully selected code snippets that showcase:

// Modern async patterns with parallel processing
await Promise.all(
  slideNotes.map(async (note, index) => {
    const slideFile = `.temp/slide-${index + 1}.txt`;
    await fs.writeFile(slideFile, note.trim());
  })
);

// Robust error handling with specific Google Cloud error codes
if (error.code === 7) { // PERMISSION_DENIED
  throw new Error('Google Cloud authentication failed. Check your credentials.');
} else if (error.code === 8) { // RESOURCE_EXHAUSTED
  throw new Error('Google Cloud quota exceeded. Please check your billing and quotas.');
}

Tested and Ready

The presentation has been successfully generated using Marptalk itself in development mode, confirming that all speaker notes are properly formatted for audio generation. When used with Google Cloud Text-to-Speech, this will create a fully automated narrated presentation perfect for onboarding new developers or sharing knowledge about the codebase.

To generate the presentation:

node src/generate.js code-walkthrough.md

This addresses the issue request for a detailed, educational code walkthrough that can be used to generate audio presentations for developers unfamiliar with the codebase.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Code walkthrough presentation</issue_title>
<issue_description>Write a marp presentation giving a code walkthrough of this project, with speaker captions in the notes.

The audience is someone unfamiliar with the codebase, and only moderately familiar with JavaScript.

We'll use this project to generate audio from the speaker captions, so be conversational and educational.

Be as detailed as you can be, I'll give feedback about where we can skip detail if it's unnecessary.

Use code blocks to highlight especially relevant bits of code.</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #4

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