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/51-notebooklm-automation/code/CLAUDE.md

@@ -0,0 +1,50 @@
+# NotebookLM Automation - Claude Code
+
+Complete programmatic control over NotebookLM notebooks, sources, and artifacts.
+
+## Prerequisites
+
+```bash
+pip install notebooklm-py
+playwright install chromium
+notebooklm login
+```
+
+## Commands
+
+### Notebooks
+```bash
+notebooklm list [--json]
+notebooklm create "Title" [--json]
+notebooklm rename <id> "New Name"
+notebooklm delete <id>
+notebooklm use <id>
+```
+
+### Sources
+```bash
+notebooklm source add "https://..." [--json]
+notebooklm source add ./file.pdf
+notebooklm source list [--json]
+notebooklm source delete <id>
+notebooklm source wait <id>
+```
+
+### Artifacts
+```bash
+notebooklm artifact list [--json]
+notebooklm artifact wait <id>
+notebooklm artifact delete <id>
+```
+
+## Environment Variables
+
+| Variable | Purpose |
+|----------|---------|
+| `NOTEBOOKLM_HOME` | Custom config dir |
+| `NOTEBOOKLM_AUTH_JSON` | Inline auth (CI/CD) |
+
+## Autonomy
+
+**Auto-run:** `list`, `status`, `create`, `use`, `source add`
+**Ask first:** `delete`, `rename`