refine(skills): lang-cypher-dev - expand pillar coverage (4/8)

## Summary

`lang-cypher-dev` has 4/8 pillar coverage and needs expansion.

## Current Coverage

- ✓ Module System
- ✓ Error Handling
- ✗ Concurrency
- ✗ Metaprogramming
- ✗ Zero/Default Values
- ✓ Serialization
- ✓ Build/Dependencies
- ✗ Testing

## Refinement Checklist

- [ ] Add Concurrency section (parallel queries, transactions, APOC async)
- [ ] Add Metaprogramming section (dynamic queries, APOC.cypher.run)
- [ ] Add Zero/Default Values section (null handling, COALESCE, default properties)
- [ ] Add Testing section (Cypher testing frameworks, Neo4j test containers)
- [ ] Run `just validate-pillars lang-cypher-dev` to verify coverage
- [ ] Ensure SKILL.md is under 500 lines (use progressive disclosure)

## Validation Command

```bash
just validate-pillars lang-cypher-dev
```