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/50-notebooklm-agent/README.md

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+# NotebookLM Agent
+
+Q&A agent that answers user questions based on NotebookLM notebooks. Leverages NotebookLM's Gemini-powered analysis to provide grounded, citation-backed responses from uploaded sources.
+
+## Use Cases
+
+- Ask questions about documents in a specific notebook
+- Get summaries with source citations
+- Compare information across multiple sources
+- Research assistant workflows
+
+## Prerequisites
+
+```bash
+pip install notebooklm-py
+playwright install chromium
+notebooklm login  # One-time browser auth
+```
+
+## Quick Start
+
+```bash
+# Set active notebook
+notebooklm use <notebook_id>
+
+# Ask questions
+notebooklm ask "What are the key findings?"
+notebooklm ask "Compare approaches from different sources"
+
+# Get citations with JSON output
+notebooklm ask "Summarize the methodology" --json
+```
+
+## Versions
+
+| Version | Path | Purpose |
+|---------|------|---------|
+| Claude Code | `code/` | CLAUDE.md for Claude Code integration |
+| Claude Desktop | `desktop/` | SKILL.md for Claude Desktop skills |
+
+## Related Skills
+
+- [51-notebooklm-automation](../51-notebooklm-automation/) - Full automation workflows
+- [52-notebooklm-studio](../52-notebooklm-studio/) - Content generation (podcasts, videos)
+- [53-notebooklm-research](../53-notebooklm-research/) - Research and source discovery