refine(skills): lang-graphql-dev - expand pillar coverage (5/8)

## Summary

`lang-graphql-dev` has 5/8 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 (schema stitching, federation, imports)
- [ ] Add Zero/Default Values section (nullable fields, default values, @default directive)
- [ ] Add Testing section (GraphQL testing tools, mocking, integration tests)
- [ ] Run `just validate-pillars lang-graphql-dev` to verify coverage
- [ ] Ensure SKILL.md is under 500 lines (use progressive disclosure)

## Validation Command

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