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/backend/checklist.md

@@ -0,0 +1,33 @@
+# Backend Implementation Checklist
+
+## Core Checklist
+- [ ] Checked existing tables/APIs via git before creating new ones
+- [ ] Database tables created in Supabase
+- [ ] Row Level Security enabled on ALL new tables
+- [ ] RLS policies created for SELECT, INSERT, UPDATE, DELETE
+- [ ] Indexes created on performance-critical columns
+- [ ] Foreign keys set with appropriate ON DELETE behavior
+- [ ] All planned API endpoints implemented in `/src/app/api/`
+- [ ] Authentication verified (no access without valid session)
+- [ ] Input validation with Zod on all POST/PUT requests
+- [ ] Meaningful error messages with correct HTTP status codes
+- [ ] No TypeScript errors in API routes
+- [ ] All endpoints tested manually
+- [ ] No hardcoded secrets in source code
+- [ ] Frontend connected to real API endpoints
+- [ ] User has reviewed and approved
+
+## Verification (run before marking complete)
+- [ ] `npm run build` passes without errors
+- [ ] All acceptance criteria from feature spec addressed in API
+- [ ] All API endpoints return correct status codes (test with curl or browser)
+- [ ] `features/INDEX.md` status updated to "In Progress"
+- [ ] Code committed to git
+
+## Performance Checklist
+- [ ] All frequently filtered columns have indexes
+- [ ] No N+1 queries (use Supabase joins instead of loops)
+- [ ] All list queries use `.limit()`
+- [ ] Zod validation on all write endpoints
+- [ ] Slow queries cached where appropriate (optional for MVP)
+- [ ] Rate limiting on public-facing APIs (optional for MVP)