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/code/CLAUDE.md

@@ -0,0 +1,55 @@
+# NotebookLM Agent - Claude Code
+
+Q&A agent that answers questions using NotebookLM's Gemini-powered analysis with source citations.
+
+## Prerequisites
+
+```bash
+pip install notebooklm-py
+playwright install chromium
+notebooklm login  # One-time auth
+```
+
+## Commands
+
+```bash
+# List notebooks
+notebooklm list
+
+# Set context
+notebooklm use <notebook_id>
+
+# Ask questions
+notebooklm ask "What are the key findings?"
+notebooklm ask "Elaborate on point 2"  # continues conversation
+notebooklm ask "New topic" --new       # new conversation
+
+# With citations (JSON output)
+notebooklm ask "Summarize" --json
+
+# Query specific sources
+notebooklm ask "Compare" -s source1 -s source2
+```
+
+## Autonomy
+
+**Auto-run:** `list`, `status`, `source list`, `ask`
+**Ask first:** `delete`, `source add`
+
+## JSON Output Format
+
+```json
+{
+  "answer": "Response with [1] [2] citations",
+  "references": [
+    {"source_id": "...", "citation_number": 1, "cited_text": "..."}
+  ]
+}
+```
+
+## Error Recovery
+
+| Error | Fix |
+|-------|-----|
+| No context | `notebooklm use <id>` |
+| Auth error | `notebooklm login` |