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/code/CLAUDE.md

@@ -0,0 +1,59 @@
+# NotebookLM Research - Claude Code
+
+Research workflows: web research, Drive search, auto-import, source extraction.
+
+## Prerequisites
+
+```bash
+pip install notebooklm-py
+playwright install chromium
+notebooklm login
+```
+
+## Research Commands
+
+```bash
+# Web research
+notebooklm source add-research "topic"
+notebooklm source add-research "topic" --mode deep --import-all
+notebooklm source add-research "topic" --mode deep --no-wait
+
+# Drive research
+notebooklm source add-research "topic" --from drive
+
+# Status and wait
+notebooklm research status
+notebooklm research wait --import-all
+```
+
+## Source Extraction
+
+```bash
+notebooklm source fulltext <id>
+notebooklm source guide <id>
+```
+
+## Research Modes
+
+| Mode | Sources | Time |
+|------|---------|------|
+| `fast` | 5-10 | seconds |
+| `deep` | 20+ | 2-5 min |
+
+## Subagent Pattern
+
+```python
+# Non-blocking deep research
+notebooklm source add-research "topic" --mode deep --no-wait
+
+# Spawn subagent to wait
+Task(
+  prompt="Wait for research and import: notebooklm research wait -n {id} --import-all",
+  subagent_type="general-purpose"
+)
+```
+
+## Autonomy
+
+**Auto-run:** `research status`, `source fulltext`, `source guide`
+**Ask first:** `source add-research`, `research wait --import-all`