Update docs, skill counts, and UX/setup-engine skills for v0.4.0

- Update skill count to 66 and hook count to 12 across README, skills-reference, and badges
- Add QA & Testing section to README and skills-reference (qa-plan, smoke-check, soak-test, etc.)
- Add Input & Platform section to technical-preferences.md (populated by /setup-engine)
- Document post-compact, notify, and validate-skill-change hooks in hooks-reference
- Expand /setup-engine, /ux-design, /ux-review, /design-system skills with input/platform context
- Restructure workflow-catalog: move UX steps into pre-production phase, improve descriptions
- Expand UPGRADING.md and WORKFLOW-GUIDE.md with v0.4.0 guidance
- Add skill-flow-diagrams examples and CODEOWNERS

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/CODEOWNERS

@@ -0,0 +1,16 @@
+# CODEOWNERS — auto-assigns reviewers to PRs by path
+# Format: <pattern> <@github-username or @org/team>
+# Last match wins. Docs: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners
+
+# Default owner for everything
+*                           @Donchitos
+
+# Core configuration — owner review required
+CLAUDE.md                   @Donchitos
+.claude/                    @Donchitos
+settings.json               @Donchitos
+
+# Skills and agents — high-impact, review carefully
+.claude/skills/             @Donchitos
+.claude/agents/             @Donchitos
+.claude/hooks/              @Donchitos