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/51-notebooklm-automation/README.md

@@ -0,0 +1,46 @@
+# NotebookLM Automation
+
+Complete automation toolkit for NotebookLM operations. Manage notebooks, sources, and artifacts programmatically.
+
+## Use Cases
+
+- Batch notebook management
+- Automated source ingestion
+- CI/CD integration for documentation
+- Multi-account workflows
+
+## Prerequisites
+
+```bash
+pip install notebooklm-py
+playwright install chromium
+notebooklm login
+```
+
+## Quick Start
+
+```bash
+# Notebook management
+notebooklm create "Project Research"
+notebooklm list --json
+notebooklm rename <id> "New Name"
+notebooklm delete <id>
+
+# Source management
+notebooklm source add "https://example.com"
+notebooklm source add ./document.pdf
+notebooklm source list --json
+```
+
+## 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
+- [52-notebooklm-studio](../52-notebooklm-studio/) - Content generation
+- [53-notebooklm-research](../53-notebooklm-research/) - Research workflows