feat(animation): auto-install motion lib + detection across init/onboard/advisor

Add lib/animation-lib-check.sh with detect_anim_eligibility,
is_anim_lib_installed and recommend_anim_install_cmd helpers.
Wire it into the framework:
- init-project STEP 5e: silent auto-install after scaffold validated
- onboard STEP 2.5: propose + wait for user confirmation (opt-in on
  existing projects)
- plugin-advisor PHASE 1/2/3: read-only detection only, never installs
- scaffolder PHASE 4: clarifies boundary (orchestrator owns motion install)
- design-gate filesystem signals: motion / motion-v / framer-motion /
  gsap / lottie-react / react-spring / popmotion / auto-animate

Recommends `motion` (rebranded from framer-motion in Nov 2024) for
React-family and Svelte stacks, `motion-v` for Vue 3 / Nuxt. Excludes
React Native (use react-native-reanimated), backend, embedded, Flutter.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/design-gate.md

@@ -21,6 +21,7 @@ Check BOTH the task description AND the filesystem:
 - `tailwind.config` or `postcss.config` present in project root
 - `tokens/`, `theme/`, or `design-system/` directory exists
 - Storybook config (`.storybook/`) present
+- Animation lib in `package.json` deps: `motion`, `motion-v`, `framer-motion` (legacy), `gsap`, `@gsap/react`, `lottie-react`, `react-spring`, `popmotion`, `@formkit/auto-animate`
 
 ## DECISION