feat: add douyin-knowledge-animation skill

[li872]

What

Add community skill douyin-knowledge-animation (zh + en entries).

It turns any knowledge topic into a Douyin-style vertical (720x1280) pure-animation explainer video: suspense cover + one punchline per animated page + freeze-frame ending, delivered as a single-file HTML Canvas animation with built-in one-key cover image export (4:3 + 3:4 PNG). Users screen-record it and add beat-synced BGM in a video editor.

Why

• Built and iterated entirely with Qoder; readability rules (Douyin safe area, min font size, pure-white text) come from real publishing practice
• Skill repo follows the Qoder marketplace skill spec: SKILL.md with name/description/version frontmatter + references/ directory for progressive disclosure

Checklist

• Frontmatter follows the skills collection schema (category: marketing, roles: content/marketer)
• Both zh and en entries provided
• installCommand tested: clone + copy to ~/.qoder/skills/