Add promotional video skill for product-level UI animations

[xiaoyan648]

Apart from the basic technical skills, I also want to add skills that can handle different scenarios. For instance, recently when I was creating promotional and instructional videos for a website, I found that using Remotion to simulate the website was extremely efficient.

Summary

• Add a new promo skill (skills/promo/SKILL.md) for creating product-level promotional videos and marketing animations
• Covers mandatory workflow (read real frontend code before recreating UI), pixel-perfect UI recreation guidelines, interaction simulation (fake cursors, typing effects, loading states, camera work), and frame-driven timeline patterns
• Uses Remotion native interpolate + Easing API consistent with the existing remotion skill, with cross-references to shared rules

Test plan

• Verify skills/promo/SKILL.md renders correctly and cross-references to ../remotion/SKILL.md and ../remotion/rules/timing.md resolve properly
• Test that an agent can discover and load this skill when given a promo-video-related prompt