feat: Migrate from agent markdown files to Skills, Rules, and Sub-Agents

Replace the manual "read .claude/agents/*.md" workflow with native
Claude Code features for a more efficient, scalable development experience:

- **Skills** (.claude/skills/): 7 auto-discovered slash commands
  (/requirements, /architecture, /frontend, /backend, /qa, /deploy, /help)
  with forked sub-agents for heavy tasks and inline execution for interactive ones
- **Rules** (.claude/rules/): 4 modular rule files (general, frontend, backend,
  security) auto-applied based on file context
- **Sub-Agents** (.claude/agents/): Lightweight configs for frontend-dev,
  backend-dev, and qa-engineer with model, tool, and turn limit settings
- **Context Engineering**: Layered context loading, context isolation via
  forked skills, built-in context recovery after compaction, and
  "always read, never guess" rules to prevent hallucinated code references
- **CLAUDE.md**: Auto-loaded project context replacing PROJECT_CONTEXT.md
- **Feature tracking**: features/INDEX.md as persistent state across sessions
- **Production guides**: docs/production/ for error tracking, security,
  performance, database optimization, and rate limiting
- **Init Mode**: /requirements detects empty PRD and bootstraps full project
  setup (PRD + all feature specs) from a single project description

Removed: 6 monolithic agent files, PROJECT_CONTEXT.md, HOW_TO_USE_AGENTS.md,
TEMPLATE_CHANGELOG.md

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

.claude/skills/frontend/checklist.md

@@ -0,0 +1,38 @@
+# Frontend Implementation Checklist
+
+Before marking frontend as complete:
+
+## shadcn/ui
+- [ ] Checked shadcn/ui for EVERY UI component needed
+- [ ] No custom duplicates of shadcn components created
+- [ ] Missing shadcn components installed via `npx shadcn@latest add`
+
+## Existing Code
+- [ ] Checked existing project components via `git ls-files src/components/`
+- [ ] Reused existing components where possible
+
+## Design
+- [ ] Design preferences clarified with user (if no mockups)
+- [ ] Component architecture from Solution Architect followed
+
+## Implementation
+- [ ] All planned components implemented
+- [ ] All components use Tailwind CSS (no inline styles, no CSS modules)
+- [ ] Loading states implemented (spinner/skeleton during data fetches)
+- [ ] Error states implemented (user-friendly error messages)
+- [ ] Empty states implemented ("No data yet" messages)
+
+## Quality
+- [ ] Responsive: Mobile (375px), Tablet (768px), Desktop (1440px)
+- [ ] Accessibility: Semantic HTML, ARIA labels, keyboard navigation
+- [ ] TypeScript: No errors (`npm run build` passes)
+- [ ] ESLint: No warnings (`npm run lint`)
+
+## Verification (run before marking complete)
+- [ ] `npm run build` passes without errors
+- [ ] All acceptance criteria from feature spec addressed in UI
+- [ ] `features/INDEX.md` status updated to "In Progress"
+
+## Completion
+- [ ] User has reviewed and approved the UI in browser
+- [ ] Code committed to git