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/settings.json

@@ -0,0 +1,22 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(npm install)",
+      "Bash(npm install:*)",
+      "Bash(npm run dev:*)",
+      "Bash(npm run build:*)",
+      "Bash(npm run lint:*)",
+      "Bash(npm run start:*)",
+      "Bash(git commit:*)",
+      "Bash(git push:*)",
+      "Bash(git tag:*)",
+      "Bash(git log:*)",
+      "Bash(git ls-files:*)",
+      "Bash(lsof:*)",
+      "Bash(kill:*)",
+      "Bash(npx shadcn@latest add:*)",
+      "Bash(ls:*)",
+      "Bash(cat:*)"
+    ]
+  }
+}