refine(skills): lang-sql-dev - expand pillar coverage (2/8)

## Summary

`lang-sql-dev` has 2/8 pillar coverage and needs expansion.

## Current Coverage

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

## Refinement Checklist

- [ ] Add Module System section (schemas, namespaces, views)
- [ ] Add Concurrency section (transactions, locking, isolation levels)
- [ ] Add Metaprogramming section (dynamic SQL, stored procedures)
- [ ] Add Zero/Default Values section (NULL handling, COALESCE, DEFAULT)
- [ ] Add Build/Dependencies section (migrations, versioning tools)
- [ ] Add Testing section (unit tests, fixtures, test databases)
- [ ] Run `just validate-pillars lang-sql-dev` to verify coverage
- [ ] Ensure SKILL.md is under 500 lines (use progressive disclosure)

## Validation Command

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