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/desktop/SKILL.md

@@ -0,0 +1,118 @@
+---
+name: notebooklm-agent
+description: |
+  Q&A agent for NotebookLM notebooks. Ask questions and get grounded, citation-backed answers from your sources.
+  Triggers: ask NotebookLM, query notebook, research question, 노트북 질문, NotebookLM 에이전트.
+---
+
+# NotebookLM Agent
+
+Q&A agent that answers questions using NotebookLM's Gemini-powered analysis. Returns grounded responses with source citations.
+
+## Prerequisites
+
+NotebookLM CLI must be installed and authenticated:
+
+```bash
+pip install notebooklm-py
+playwright install chromium
+notebooklm login
+```
+
+## When This Skill Activates
+
+- User asks "ask NotebookLM about X"
+- User wants to "query my notebook"
+- User needs "research answers from sources"
+- Korean: "노트북LM에서 찾아줘", "노트북 질문"
+
+## Quick Reference
+
+| Task | Command |
+|------|---------|
+| List notebooks | `notebooklm list` |
+| Set active notebook | `notebooklm use <id>` |
+| Ask question | `notebooklm ask "question"` |
+| New conversation | `notebooklm ask "question" --new` |
+| With citations | `notebooklm ask "question" --json` |
+| Specific sources | `notebooklm ask "q" -s src1 -s src2` |
+
+## Workflow
+
+### 1. Select Notebook
+
+```bash
+# List available notebooks
+notebooklm list
+
+# Set context (use partial ID)
+notebooklm use abc123
+```
+
+### 2. Ask Questions
+
+```bash
+# Simple question
+notebooklm ask "What are the main findings?"
+
+# Follow-up (continues conversation)
+notebooklm ask "Can you elaborate on point 2?"
+
+# New conversation
+notebooklm ask "Different topic" --new
+
+# Query specific sources only
+notebooklm ask "Compare these two" -s source1_id -s source2_id
+```
+
+### 3. Get Structured Output
+
+For citations and references, use `--json`:
+
+```bash
+notebooklm ask "Summarize the methodology" --json
+```
+
+Returns:
+```json
+{
+  "answer": "The methodology involves... [1] [2]",
+  "references": [
+    {"source_id": "abc...", "citation_number": 1, "cited_text": "..."},
+    {"source_id": "def...", "citation_number": 2, "cited_text": "..."}
+  ]
+}
+```
+
+## Autonomy Rules
+
+**Run automatically:**
+- `notebooklm list` - view notebooks
+- `notebooklm status` - check context
+- `notebooklm source list` - view sources
+- `notebooklm ask "..."` - answer questions
+
+**Ask before running:**
+- `notebooklm delete` - destructive operations
+- `notebooklm source add` - modifies notebook
+
+## Tips
+
+1. **Set context first**: Always `use` a notebook before asking
+2. **Use --json for citations**: Get structured references for research
+3. **Continue conversations**: Omit `--new` for follow-up questions
+4. **Filter sources**: Use `-s` to query specific documents only
+
+## Error Handling
+
+| Error | Solution |
+|-------|----------|
+| "No notebook context" | Run `notebooklm use <id>` |
+| Auth error | Run `notebooklm login` |
+| Source not found | Check `notebooklm source list` |
+
+## Related Skills
+
+- **notebooklm-automation**: Full notebook management
+- **notebooklm-studio**: Generate podcasts, videos, quizzes
+- **notebooklm-research**: Add sources and research workflows