feat(skills): add design-motion-principles from kylezantos

Motion/animation design skill with 3-designer lens (Emil Kowalski,
Jakub Krehel, Jhey Tompkins). Two modes: Create and Audit.
Complements frontend-design (broad) with deep motion expertise.

Integration points:
- skills-external/design-motion-principles/ — skill files + references
- link.sh EXTERNAL_SKILLS — symlink to ~/.claude/skills/
- install-plugins.sh step 8c — presence check
- update-all.sh step 7.2 — sync from GitHub
- profiles: design, full, web, web-full — listed as external
- plugin-advisor — decision table, recommended sets, conditional rules
- design-gate — symlink check + non-blocking warning if missing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

skills-external/design-motion-principles/references/accessibility.md

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+# Accessibility
+
+**This is not optional.** Motion can cause discomfort, nausea, or distraction for many users.
+
+---
+
+## Respect User Preferences
+
+```css
+@media (prefers-reduced-motion: reduce) {
+  *,
+  *::before,
+  *::after {
+    animation-duration: 0.01ms !important;
+    animation-iteration-count: 1 !important;
+    transition-duration: 0.01ms !important;
+    scroll-behavior: auto !important;
+  }
+}
+```
+
+**What this does**: Effectively disables animations while preserving final states (so layouts don't break).
+
+---
+
+## Functional vs. Decorative Motion
+
+| Type | Purpose | Reduced Motion Behavior |
+|------|---------|------------------------|
+| **Functional** | Indicates state changes, spatial relationships, orientation | May need alternative (instant state change, no transition) |
+| **Decorative** | Pure delight, visual interest | Can be fully removed |
+
+**The test**: Does removing this animation break the user's ability to understand what happened? If yes, it's functional.
+
+---
+
+## Motion Sensitivity Considerations
+
+- Avoid large-scale motion (full-screen transitions, parallax)
+- Avoid continuous or looping animations that can't be paused
+- Provide pause controls for any ambient animation
+- Be especially careful with vestibular triggers: zooming, spinning, parallax
+
+---
+
+## Implementation Checklist
+
+- [ ] Tested with `prefers-reduced-motion: reduce` enabled
+- [ ] No vestibular triggers (excessive zoom, spin, parallax)
+- [ ] Looping animations can be paused
+- [ ] Functional animations have non-motion alternatives
+- [ ] Users can complete all tasks with animations disabled