feat(skills): expand lang skills with missing pillars (7 skills need work)

## Summary

Seven `lang-*-dev` skills have pillar coverage below the recommended 6/8 threshold. These need expansion before they can support conversion skills.

## Skills Below Threshold

| Skill | Coverage | Missing Pillars |
|-------|----------|-----------------|
| `lang-rust-errors-dev` | 1/8 | Module, Concurrency, Metaprogramming, Zero/Default, Serialization, Build, Testing |
| `lang-sparql-dev` | 2/8 | Module, Concurrency, Metaprogramming, Zero/Default, Serialization, Testing |
| `lang-sql-dev` | 2/8 | Module, Concurrency, Metaprogramming, Zero/Default, Serialization, Testing |
| `lang-cypher-dev` | 4/8 | Concurrency, Metaprogramming, Zero/Default, Testing |
| `lang-graphql-dev` | 5/8 | Module, Zero/Default, Testing |
| `lang-rust-cargo-dev` | 5/8 | Error, Concurrency, Metaprogramming |
| `lang-rust-docs-dev` | 5/8 | Module, Concurrency, Metaprogramming |

## 8 Pillars Reference

1. **Module System** - imports, exports, namespaces, packages
2. **Error Handling** - Result, Option, exceptions
3. **Concurrency** - async, threads, actors, channels
4. **Metaprogramming** - macros, decorators, reflection
5. **Zero/Default Values** - null, nil, None, Option
6. **Serialization** - JSON, serde, codecs
7. **Build/Dependencies** - package managers, build tools
8. **Testing** - unit tests, property tests, frameworks

## Validation Command

```bash
just validate-pillars
just validate-all-lang-skills
```

## Priority

High priority for:
- `lang-rust-errors-dev` (1/8) - focus skill needs refactoring or merging
- `lang-sparql-dev`, `lang-sql-dev` (2/8) - query languages with significant gaps

Lower priority (5/8 - acceptable but not complete):
- `lang-rust-cargo-dev`, `lang-rust-docs-dev` - specialized focus skills
- `lang-graphql-dev` - schema language

## Notes

Some specialized skills (like `lang-rust-errors-dev`) may be candidates for merging into parent skills rather than expansion.