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/general.md

@@ -0,0 +1,37 @@
+# General Project Rules
+
+## Feature Tracking
+- All features are tracked in `features/INDEX.md` - read it before starting any work
+- Feature specs live in `features/PROJ-X-feature-name.md`
+- Feature IDs are sequential: check INDEX.md for the next available number
+- One feature per spec file (Single Responsibility)
+- Never combine multiple independent functionalities in one spec
+
+## Git Conventions
+- Commit format: `type(PROJ-X): description`
+- Types: feat, fix, refactor, test, docs, deploy, chore
+- Check existing features before creating new ones: `ls features/ | grep PROJ-`
+- Check existing components before building: `git ls-files src/components/`
+- Check existing APIs before building: `git ls-files src/app/api/`
+
+## Human-in-the-Loop
+- Always ask for user approval before finalizing deliverables
+- Present options using clear choices rather than open-ended questions
+- Never proceed to the next workflow phase without user confirmation
+
+## Status Updates
+- Update `features/INDEX.md` when feature status changes
+- Update the feature spec header status field
+- Valid statuses: Planned, In Progress, In Review, Deployed
+
+## File Handling
+- ALWAYS read a file before modifying it - never assume contents from memory
+- After context compaction, re-read files before continuing work
+- When unsure about current project state, read `features/INDEX.md` first
+- Run `git diff` to verify what has already been changed in this session
+- Never guess at import paths, component names, or API routes - verify by reading
+
+## Handoffs Between Skills
+- After completing a skill, suggest the next skill to the user
+- Format: "Next step: Run `/skillname` to [action]"
+- Handoffs are always user-initiated, never automatic