feat: Add NotebookLM skills (50-53) for Claude Code and Desktop

Add 4 specialized NotebookLM skills based on notebooklm-py library:

- 50-notebooklm-agent: Q&A agent for notebook queries with citations
- 51-notebooklm-automation: Full notebook/source/artifact management
- 52-notebooklm-studio: Content generation (podcasts, videos, quizzes)
- 53-notebooklm-research: Web/Drive research and source discovery

Each skill includes:
- README.md: Overview and quick start
- code/CLAUDE.md: Claude Code version (concise)
- desktop/SKILL.md: Claude Desktop version (with YAML frontmatter)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

custom-skills/52-notebooklm-studio/README.md

@@ -0,0 +1,59 @@
+# NotebookLM Studio
+
+Content generation toolkit for NotebookLM Studio artifacts. Create podcasts, videos, quizzes, flashcards, slide decks, infographics, and more.
+
+## Use Cases
+
+- Generate audio overviews (podcasts) from sources
+- Create educational videos with multiple styles
+- Build quizzes and flashcards for learning
+- Export mind maps and data tables
+
+## Prerequisites
+
+```bash
+pip install notebooklm-py
+playwright install chromium
+notebooklm login
+```
+
+## Quick Start
+
+```bash
+# Generate content
+notebooklm generate audio "Focus on key findings"
+notebooklm generate video --style whiteboard
+notebooklm generate quiz --difficulty medium
+notebooklm generate flashcards
+
+# Download artifacts
+notebooklm download audio ./podcast.mp3
+notebooklm download video ./overview.mp4
+notebooklm download quiz --format markdown ./quiz.md
+```
+
+## Content Types
+
+| Type | Formats | Output |
+|------|---------|--------|
+| Audio | deep-dive, brief, critique, debate | MP3 |
+| Video | 9 visual styles | MP4 |
+| Quiz | configurable difficulty | JSON/MD/HTML |
+| Flashcards | configurable quantity | JSON/MD/HTML |
+| Slide Deck | detailed, presenter | PDF |
+| Infographic | 3 orientations | PNG |
+| Mind Map | hierarchical | JSON |
+| Data Table | custom structure | CSV |
+
+## Versions
+
+| Version | Path | Purpose |
+|---------|------|---------|
+| Claude Code | `code/` | CLAUDE.md for Claude Code |
+| Claude Desktop | `desktop/` | SKILL.md for Claude Desktop |
+
+## Related Skills
+
+- [50-notebooklm-agent](../50-notebooklm-agent/) - Q&A agent
+- [51-notebooklm-automation](../51-notebooklm-automation/) - Notebook management
+- [53-notebooklm-research](../53-notebooklm-research/) - Research workflows