review(skills): lang-csharp-dev

## Skill Review

**Skill**: `lang-csharp-dev`
**Category**: lang

## Review Checklist

### Structure
- [ ] SKILL.md exists and has valid frontmatter
- [ ] SKILL.md is under 500 lines (use progressive disclosure if needed)
- [ ] Has Overview section
- [ ] Has Quick Reference section
- [ ] Has Troubleshooting section

### 8 Pillars Coverage
- [ ] Module System (imports, exports, namespaces)
- [ ] Error Handling (Result, Option, exceptions)
- [ ] Concurrency (async, threads, actors)
- [ ] Metaprogramming (macros, decorators, reflection)
- [ ] Zero/Default Values (null, nil, None handling)
- [ ] Serialization (JSON, serde, codecs)
- [ ] Build/Dependencies (package managers, build tools)
- [ ] Testing (unit tests, property tests, frameworks)

### Quality
- [ ] Description is action-oriented and under 200 chars
- [ ] Examples are included where helpful
- [ ] No time-sensitive information
- [ ] Consistent terminology throughout

## Validation Command

```bash
just validate-skill components/skills/lang-csharp-dev
just validate-pillars lang-csharp-dev
```