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>

docs/PRD.md

@@ -0,0 +1,29 @@
+# Product Requirements Document
+
+## Vision
+_Describe what you are building and why._
+
+## Target Users
+_Who will use this product? Describe their needs and pain points._
+
+## Core Features (Roadmap)
+
+| Priority | Feature | Status |
+|----------|---------|--------|
+| P0 (MVP) | _Feature 1_ | Planned |
+| P0 (MVP) | _Feature 2_ | Planned |
+| P1 | _Feature 3_ | Planned |
+| P2 | _Feature 4_ | Planned |
+
+## Success Metrics
+_How will you measure success? (e.g., user signups, retention, task completion rate)_
+
+## Constraints
+_Budget, timeline, technical limitations, team size._
+
+## Non-Goals
+_What are you explicitly NOT building in this version?_
+
+---
+
+Use `/requirements` to create detailed feature specifications for each item in the roadmap above.