feat: initial recipes repo with identity-cat

- Recipe schema using artdag primitives (SOURCE, EFFECT)
- identity-cat: applies identity effect to cat image
- Test verifying identity property: output_hash == input_hash
- Demonstrates content-addressed DAG execution

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>

.gitignore

@@ -0,0 +1,4 @@
+__pycache__/
+*.py[cod]
+.env
+.venv