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>

link.sh

@@ -50,7 +50,7 @@ if [ ! -d "$REPO/skills-external/gstack" ]; then
   echo "⚠️  GStack submodule not found — run: git submodule update --init"
 fi
 
-EXTERNAL_SKILLS=(emil-design-eng frontend-design)
+EXTERNAL_SKILLS=(emil-design-eng frontend-design design-motion-principles)
 for _ext_skill in "${EXTERNAL_SKILLS[@]}"; do
   if [ -d "$REPO/skills-external/$_ext_skill" ]; then
     if [ -L "$CLAUDE/skills/$_ext_skill" ] && [ "$(readlink "$CLAUDE/skills/$_ext_skill")" = "$REPO/skills-external/$_ext_skill" ]; then