[FEATURE] Enhanced doctor command validation

## ✨ Feature Request Template

### **Feature Description**
Enhance the `doctor` command with deeper validation checks beyond basic configuration syntax.

### **Use Case**
As new features are added (LLM integration, external services), users need a comprehensive health check to verify all components are working properly.

### **Proposed Solution**
Add validation rules with severity levels:
- **ERROR**: Blocking issues that prevent functionality
- **WARNING**: Missing optional features or recommended settings
- **INFO**: Status information

Example output:
```
✅ Configuration: PASS
⚠️ LLM API: WARNING (no API key configured)
✅ Permissions: PASS
```

Rules handle missing validation logic gracefully (show WARNING instead of failing).

### **Alternatives Considered**
- Keep basic validation only: Limited future utility
- Separate commands: Fragments user experience

### **Acceptance Criteria**
- [ ] Support ERROR/WARNING/INFO severity levels with clear indicators
- [ ] Add framework for future LLM/API validation rules
- [ ] Graceful handling when validation logic is unavailable
- [ ] Fast execution for basic checks
- [ ] Comprehensive test coverage

### **Additional Context**
Sets foundation for validating future LLM and external service integrations. Current configuration validation becomes part of this expanded framework.