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/requirements/template.md

@@ -0,0 +1,36 @@
+# PROJ-X: Feature Name
+
+## Status: Planned
+**Created:** YYYY-MM-DD
+**Last Updated:** YYYY-MM-DD
+
+## Dependencies
+- None
+
+## User Stories
+- As a [user type], I want to [action] so that [goal]
+
+## Acceptance Criteria
+- [ ] Criterion 1
+- [ ] Criterion 2
+
+## Edge Cases
+- What happens when...?
+- How do we handle...?
+
+## Technical Requirements (optional)
+- Performance: < 200ms response time
+- Security: Authentication required
+- Browser Support: Chrome, Firefox, Safari
+
+---
+<!-- Sections below are added by subsequent skills -->
+
+## Tech Design (Solution Architect)
+_To be added by /architecture_
+
+## QA Test Results
+_To be added by /qa_
+
+## Deployment
+_To be added by /deploy_