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.

.gitignore

@@ -51,4 +51,6 @@ node_modules/
 
 # Remotion build output and copied project assets (regenerable at render time)
 remotion-composer/out/
-remotion-composer/public/
+remotion-composer/public/*
+# But keep demo props (shipped with the project for zero-key demos)
+!remotion-composer/public/demo-props/