Zero-key video formula: fix rendering bugs, add 8 demo compositions, update skills

Fix ComparisonCard and CalloutBox dark-theme passthrough in SceneRenderer.
Add 8 demo-props JSON files (3 fixed originals + 5 new compositions).
Update scene-director and compose-director skills with all 10 Remotion
scene types and zero-key rendering guidance. Document the proven
all-dark-background formula in remotion.md. Add cinematic renderer,
onboarding skill, prompt gallery, and demo render script.

Makefile

@@ -1,4 +1,4 @@
-.PHONY: setup install install-dev install-gpu test test-contracts lint clean preflight
+.PHONY: setup install install-dev install-gpu test test-contracts lint clean preflight demo
 
 # ---- One-command setup ----
 
@@ -48,6 +48,15 @@ test-contracts:
 preflight:
 	python -c "from tools.tool_registry import registry; import json; registry.discover(); print(json.dumps(registry.provider_menu(), indent=2))"
 
+demo:
+	@echo "==> Rendering zero-key demo videos (no API keys needed)..."
+	@echo "    These use only Remotion components — animated charts, text, data viz."
+	@echo ""
+	./render-demo.sh
+
+demo-list:
+	@./render-demo.sh --list
+
 lint:
 	python -m py_compile tools/base_tool.py
 	python -m py_compile tools/tool_registry.py