feat: add dog effect

- Ignores input, returns dog.mkv from immutable URL
- Downloads from art-source with hash verification
- Caches downloaded file locally
- 5 automated tests (all passing)

Owner: @giles@artdag.rose-ash.com

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -0,0 +1,35 @@
+# Art DAG - Effects
+
+Custom effects for the Art DAG engine.
+
+## Effects
+
+| Effect | Description | Output Hash |
+|--------|-------------|-------------|
+| dog | Returns dog.mkv regardless of input | `772f26f9...` |
+
+## Structure
+
+```
+dog/
+├── README.md         # Documentation
+├── requirements.txt  # Dependencies
+├── effect.py         # Implementation
+└── test_effect.py    # Automated tests
+```
+
+## Running Tests
+
+```bash
+cd dog && python -m pytest test_effect.py -v
+```
+
+## Related
+
+- **Engine**: https://github.com/gilesbradshaw/art-dag
+- **Registry**: https://git.rose-ash.com/art-dag/registry
+- **Recipes**: https://git.rose-ash.com/art-dag/recipes
+
+## Owner
+
+`@giles@artdag.rose-ash.com`