Add design-first workflow aligned with opencode boilerplate

- New commands: /define-design, /update-design, /generate-app (web | flutter | winui3)
- New skills: ui-design, design-tokens, platform-ui-generation
- /add-feature: check design spec before implementation (Step 3.5) and reconcile all six persistent docs after implementation (Step 8)
- /setup-project: automatic document creation with self-check checkpoints
- CLAUDE.md: document docs/design/, .claude/ configuration, and the design phase in the development process
- Enrich subagent descriptions; allowlist new skills in settings.json
- Add docs/design/README.md

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.claude/settings.json

@@ -6,7 +6,11 @@
       "Skill(architecture-design)",
       "Skill(repository-structure)",
       "Skill(development-guidelines)",
-      "Skill(glossary-creation)"
+      "Skill(glossary-creation)",
+      "Skill(ui-design)",
+      "Skill(design-tokens)",
+      "Skill(platform-ui-generation)",
+      "Skill(steering)"
     ],
     "deny": [],
     "ask": []