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/53-notebooklm-research/README.md

@@ -0,0 +1,53 @@
+# NotebookLM Research
+
+Research and source discovery toolkit for NotebookLM. Web research, Drive search, auto-import, and source text extraction.
+
+## Use Cases
+
+- Automated research on topics
+- Web and Google Drive source discovery
+- Source fulltext extraction
+- Research pipeline automation
+
+## Prerequisites
+
+```bash
+pip install notebooklm-py
+playwright install chromium
+notebooklm login
+```
+
+## Quick Start
+
+```bash
+# Web research
+notebooklm source add-research "topic query"
+notebooklm source add-research "topic" --mode deep --import-all
+
+# Drive research
+notebooklm source add-research "topic" --from drive
+
+# Extract source content
+notebooklm source fulltext <source_id>
+notebooklm source guide <source_id>
+```
+
+## Research Modes
+
+| Mode | Sources | Time |
+|------|---------|------|
+| `fast` | 5-10 | seconds |
+| `deep` | 20+ | 2-5 min |
+
+## 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
+- [52-notebooklm-studio](../52-notebooklm-studio/) - Content generation