Motion Story compatibility entry

The maintained skill is now eric-intro-motion-skill. Read that entry and use its runtime. New commands and recipes live in ../eric-intro-motion-skill/. This alias is retained for older prompts; social-motion remains the companion ratio tool.

Old scripts/cli.mjs, scripts/compose.mjs and scripts/assemble-media.mjs commands forward to the current runtime. Update old asset paths to eric-intro-motion-skill/assets/. Do not install a second old runtime.