fix: add root-level SKILL.md and .codex/instructions.md to all domains (#301)

Root cause: CLI tools (ai-agent-skills, agent-skills-cli) look for SKILL.md
at the specified install path. 7 of 9 domain directories were missing this
file, causing "Skill not found" errors for bundle installs like:
  npx ai-agent-skills install alirezarezvani/claude-skills/engineering-team

Fix:
- Add root-level SKILL.md with YAML frontmatter to 7 domains
- Add .codex/instructions.md to 8 domains (for Codex CLI discovery)
- Update INSTALLATION.md with accurate skill counts (53→170)
- Add troubleshooting entry for "Skill not found" error

All 9 domains now have: SKILL.md + .codex/instructions.md + plugin.json

Closes #301

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

engineering/.codex/instructions.md

@@ -0,0 +1,29 @@
+# Engineering Advanced Skills — Codex CLI Instructions
+
+When working on advanced engineering tasks, use the POWERFUL-tier skill system:
+
+## Routing
+
+1. **Identify the domain:** Architecture, CI/CD, databases, observability, agents, RAG, or security
+2. **Read the specialist SKILL.md** for detailed instructions
+
+## Key Skills by Task
+
+| Task | Skill |
+|------|-------|
+| Agent architecture | agent-designer |
+| RAG systems | rag-architect |
+| MCP servers | mcp-server-builder |
+| CI/CD pipelines | ci-cd-pipeline-builder |
+| Database schema | database-designer |
+| Performance tuning | performance-profiler |
+| API review | api-design-reviewer |
+| Monitoring/SLOs | observability-designer |
+| Release management | release-manager |
+| Security audit | skill-security-auditor |
+| Tech debt | tech-debt-tracker |
+
+## Rules
+
+- Load only 1-2 skills per request — don't bulk-load
+- These are advanced skills — combine with engineering-team/ core skills as needed