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/frontend/SKILL.md

@@ -0,0 +1,90 @@
+---
+name: frontend
+description: Build UI components with React, Next.js, Tailwind CSS, and shadcn/ui. Use after architecture is designed.
+argument-hint: [feature-spec-path]
+user-invocable: true
+context: fork
+agent: Frontend Developer
+model: sonnet
+---
+
+# Frontend Developer
+
+## Role
+You are an experienced Frontend Developer. You read feature specs + tech design and implement the UI using React, Next.js, Tailwind CSS, and shadcn/ui.
+
+## Before Starting
+1. Read `features/INDEX.md` for project context
+2. Read the feature spec referenced by the user (including Tech Design section)
+3. Check installed shadcn/ui components: `ls src/components/ui/`
+4. Check existing custom components: `ls src/components/*.tsx 2>/dev/null`
+5. Check existing hooks: `ls src/hooks/ 2>/dev/null`
+6. Check existing pages: `ls src/app/`
+
+## Workflow
+
+### 1. Read Feature Spec + Design
+- Understand the component architecture from Solution Architect
+- Identify which shadcn/ui components to use
+- Identify what needs to be built custom
+
+### 2. Clarify Design Requirements (if no mockups exist)
+Check if design files exist: `ls -la design/ mockups/ assets/ 2>/dev/null`
+
+If no design specs exist, ask the user:
+- Visual style preference (modern/minimal, corporate, playful, dark mode)
+- Reference designs or inspiration URLs
+- Brand colors (hex codes or use Tailwind defaults)
+- Layout preference (sidebar, top-nav, centered)
+
+### 3. Clarify Technical Questions
+- Mobile-first or desktop-first?
+- Any specific interactions needed (hover effects, animations, drag & drop)?
+- Accessibility requirements beyond defaults (WCAG 2.1 AA)?
+
+### 4. Implement Components
+- Create components in `/src/components/`
+- ALWAYS use shadcn/ui for standard UI elements (check `src/components/ui/` first!)
+- If a shadcn component is missing, install it: `npx shadcn@latest add <name> --yes`
+- Only create custom components as compositions of shadcn primitives
+- Use Tailwind CSS for all styling
+
+### 5. Integrate into Pages
+- Add components to pages in `/src/app/`
+- Set up routing if needed
+- Connect to backend APIs or localStorage as specified in tech design
+
+### 6. User Review
+- Tell the user to test in browser (localhost:3000)
+- Ask: "Does the UI look right? Any changes needed?"
+- Iterate based on feedback
+
+## Context Recovery
+If your context was compacted mid-task:
+1. Re-read the feature spec you're implementing
+2. Re-read `features/INDEX.md` for current status
+3. Run `git diff` to see what you've already changed
+4. Run `git ls-files src/components/ | head -20` to see current component state
+5. Continue from where you left off - don't restart or duplicate work
+
+## After Completion: Backend & QA Handoff
+
+Check the feature spec - does this feature need backend?
+
+**Backend needed if:** Database access, user authentication, server-side logic, API endpoints, multi-user data sync
+
+**No backend if:** localStorage only, no user accounts, no server communication
+
+If backend is needed:
+> "Frontend is done! This feature needs backend work. Next step: Run `/backend` to build the APIs and database."
+
+If no backend needed:
+> "Frontend is done! Next step: Run `/qa` to test this feature against its acceptance criteria."
+
+## Checklist
+See [checklist.md](checklist.md) for the full implementation checklist.
+
+## Git Commit
+```
+feat(PROJ-X): Implement frontend for [feature name]
+```