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/qa/test-template.md

@@ -0,0 +1,56 @@
+# QA Test Results Template
+
+Add this section to the END of the feature spec `/features/PROJ-X.md`:
+
+```markdown
+---
+
+## QA Test Results
+
+**Tested:** YYYY-MM-DD
+**App URL:** http://localhost:3000
+**Tester:** QA Engineer (AI)
+
+### Acceptance Criteria Status
+
+#### AC-1: [Criterion Name]
+- [x] Sub-criterion passed
+- [ ] BUG: Sub-criterion failed (describe what went wrong)
+
+#### AC-2: [Criterion Name]
+- [x] All sub-criteria passed
+
+### Edge Cases Status
+
+#### EC-1: [Edge Case Name]
+- [x] Handled correctly
+
+#### EC-2: [Edge Case Name]
+- [ ] BUG: Not handled (describe expected vs actual behavior)
+
+### Security Audit Results
+- [x] Authentication: Cannot access without login
+- [x] Authorization: Users cannot access other users' data
+- [x] Input validation: XSS attempts blocked
+- [x] Rate limiting: Excessive requests handled
+- [ ] BUG: [Security issue description]
+
+### Bugs Found
+
+#### BUG-1: [Bug Title]
+- **Severity:** Critical | High | Medium | Low
+- **Steps to Reproduce:**
+  1. Go to [page]
+  2. Do [action]
+  3. Expected: [what should happen]
+  4. Actual: [what actually happens]
+- **Screenshot:** [if visual bug]
+- **Priority:** Fix before deployment | Fix in next sprint | Nice to have
+
+### Summary
+- **Acceptance Criteria:** X/Y passed
+- **Bugs Found:** N total (C critical, H high, M medium, L low)
+- **Security:** [Pass / Issues found]
+- **Production Ready:** YES / NO
+- **Recommendation:** [Deploy / Fix bugs first]
+```