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/backend.md

@@ -0,0 +1,25 @@
+# Backend Development Rules
+
+## Database (Supabase)
+- ALWAYS enable Row Level Security on every table
+- Create RLS policies for SELECT, INSERT, UPDATE, DELETE
+- Add indexes on columns used in WHERE, ORDER BY, and JOIN clauses
+- Use foreign keys with ON DELETE CASCADE where appropriate
+- Never skip RLS - security first
+
+## API Routes
+- Validate all inputs using Zod schemas before processing
+- Always check authentication: verify user session exists
+- Return meaningful error messages with appropriate HTTP status codes
+- Use `.limit()` on all list queries
+
+## Query Patterns
+- Use Supabase joins instead of N+1 query loops
+- Use `unstable_cache` from Next.js for rarely-changing data
+- Always handle errors from Supabase responses
+
+## Security
+- Never hardcode secrets in source code
+- Use environment variables for all credentials
+- Validate and sanitize all user input
+- Use parameterized queries (Supabase handles this)