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/rules/frontend.md

@@ -0,0 +1,26 @@
+# Frontend Development Rules
+
+## shadcn/ui First (MANDATORY)
+- Before creating ANY UI component, check if shadcn/ui has it: `ls src/components/ui/`
+- NEVER create custom implementations of: Button, Input, Select, Checkbox, Switch, Dialog, Modal, Alert, Toast, Table, Tabs, Card, Badge, Dropdown, Popover, Tooltip, Navigation, Sidebar, Breadcrumb
+- If a shadcn component is missing, install it: `npx shadcn@latest add <name> --yes`
+- Custom components are ONLY for business-specific compositions that internally use shadcn primitives
+
+## Import Pattern
+```tsx
+import { Button } from "@/components/ui/button"
+import { Card, CardHeader, CardTitle, CardContent } from "@/components/ui/card"
+```
+
+## Component Standards
+- Use Tailwind CSS exclusively (no inline styles, no CSS modules)
+- All components must be responsive (mobile 375px, tablet 768px, desktop 1440px)
+- Implement loading states, error states, and empty states
+- Use semantic HTML and ARIA labels for accessibility
+- Keep components small and focused
+- Use TypeScript interfaces for all props
+
+## Auth Best Practices (Supabase)
+- Use `window.location.href` for post-login redirect (not `router.push`)
+- Always verify `data.session` exists before redirecting
+- Always reset loading state in all code paths (success, error, finally)