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

engineering/SKILL.md

@@ -0,0 +1,70 @@
+---
+name: "engineering-advanced-skills"
+description: "25 advanced POWERFUL-tier engineering skills covering agent design, RAG architecture, MCP servers, CI/CD pipelines, database design, observability, security auditing, release management, and platform operations. Works with Claude Code, Codex CLI, and OpenClaw."
+version: 1.1.0
+author: Alireza Rezvani
+license: MIT
+tags:
+  - engineering
+  - architecture
+  - agents
+  - rag
+  - mcp
+  - ci-cd
+  - observability
+agents:
+  - claude-code
+  - codex-cli
+  - openclaw
+---
+
+# Engineering Advanced Skills (POWERFUL Tier)
+
+25 advanced engineering skills for complex architecture, automation, and platform operations.
+
+## Quick Start
+
+### Claude Code
+```
+/read engineering/agent-designer/SKILL.md
+```
+
+### Codex CLI
+```bash
+npx agent-skills-cli add alirezarezvani/claude-skills/engineering
+```
+
+## Skills Overview
+
+| Skill | Folder | Focus |
+|-------|--------|-------|
+| Agent Designer | `agent-designer/` | Multi-agent architecture patterns |
+| Agent Workflow Designer | `agent-workflow-designer/` | Workflow orchestration |
+| API Design Reviewer | `api-design-reviewer/` | REST/GraphQL linting, breaking changes |
+| API Test Suite Builder | `api-test-suite-builder/` | API test generation |
+| Changelog Generator | `changelog-generator/` | Automated changelogs |
+| CI/CD Pipeline Builder | `ci-cd-pipeline-builder/` | Pipeline generation |
+| Codebase Onboarding | `codebase-onboarding/` | New dev onboarding guides |
+| Database Designer | `database-designer/` | Schema design, migrations |
+| Database Schema Designer | `database-schema-designer/` | ERD, normalization |
+| Dependency Auditor | `dependency-auditor/` | Dependency security scanning |
+| Env Secrets Manager | `env-secrets-manager/` | Secrets rotation, vault |
+| Git Worktree Manager | `git-worktree-manager/` | Parallel branch workflows |
+| Interview System Designer | `interview-system-designer/` | Hiring pipeline design |
+| MCP Server Builder | `mcp-server-builder/` | MCP tool creation |
+| Migration Architect | `migration-architect/` | System migration planning |
+| Monorepo Navigator | `monorepo-navigator/` | Monorepo tooling |
+| Observability Designer | `observability-designer/` | SLOs, alerts, dashboards |
+| Performance Profiler | `performance-profiler/` | CPU, memory, load profiling |
+| PR Review Expert | `pr-review-expert/` | Pull request analysis |
+| RAG Architect | `rag-architect/` | RAG system design |
+| Release Manager | `release-manager/` | Release orchestration |
+| Runbook Generator | `runbook-generator/` | Operational runbooks |
+| Skill Security Auditor | `skill-security-auditor/` | Skill vulnerability scanning |
+| Skill Tester | `skill-tester/` | Skill quality evaluation |
+| Tech Debt Tracker | `tech-debt-tracker/` | Technical debt management |
+
+## Rules
+
+- Load only the specific skill SKILL.md you need
+- These are advanced skills — combine with engineering-team/ core skills as needed