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/code/CLAUDE.md

@@ -0,0 +1,69 @@
+# NotebookLM Studio - Claude Code
+
+Generate NotebookLM Studio content: audio, video, quizzes, flashcards, slides, infographics, mind maps.
+
+## Prerequisites
+
+```bash
+pip install notebooklm-py
+playwright install chromium
+notebooklm login
+```
+
+## Generate Commands
+
+```bash
+# Audio
+notebooklm generate audio "instructions"
+notebooklm generate audio --format debate --length longer
+
+# Video
+notebooklm generate video --style whiteboard
+
+# Quiz & Flashcards
+notebooklm generate quiz --difficulty hard
+notebooklm generate flashcards --quantity more
+
+# Visual
+notebooklm generate slide-deck --format detailed
+notebooklm generate infographic --orientation portrait
+notebooklm generate mind-map
+
+# Data
+notebooklm generate data-table "description"
+notebooklm generate report --format study_guide
+```
+
+## Download Commands
+
+```bash
+notebooklm artifact list  # Check status
+notebooklm download audio ./podcast.mp3
+notebooklm download video ./video.mp4
+notebooklm download quiz --format markdown ./quiz.md
+notebooklm download flashcards --format json ./cards.json
+notebooklm download slide-deck ./slides.pdf
+notebooklm download mind-map ./mindmap.json
+```
+
+## Video Styles
+
+`classic`, `whiteboard`, `kawaii`, `anime`, `pixel`, `watercolor`, `neon`, `paper`, `sketch`
+
+## Audio Formats
+
+`deep-dive`, `brief`, `critique`, `debate`
+
+## Timing
+
+| Type | Time |
+|------|------|
+| Mind map | Instant |
+| Quiz | 5-15 min |
+| Audio | 10-20 min |
+| Video | 15-45 min |
+
+## Autonomy
+
+**Auto-run:** `artifact list`
+**Ask first:** `generate *`, `download *`