Implement structured output variables (vars field)

## Problem
Issue #307 specifies structured output variables that are missing from the current implementation:

```yaml
steps:
- id: analyze_data
vars:
- summary: "Brief summary of analysis results"
- statistics: {"mean": 0.0, "std": 0.0, "count": 0}
- recommendations: ["List of recommended actions"]
- confidence_score: None # Default format - save as string
```

## Solution Required
Implement structured variable system with LangChain integration:

### 1. YAML Specification Support
- Parse `vars` field in step definitions
- Support format descriptions and example objects
- Variable access via `pipeline.step_name.var_name` syntax

### 2. LangChain Structured Outputs
- Generate Pydantic models from variable definitions
- Enforce output structure during model execution
- Type validation and conversion

### 3. Variable Management Integration
- Store structured outputs in existing VariableManager
- Template resolution with structured data access
- Cross-step variable dependencies

## Implementation Details

### Variable Definition Formats
```yaml
vars:
- simple_string: None # String output (default)
- formatted_text: "Markdown formatted result" # Format hint
- json_object: {"key": "value"} # JSON structure example
- typed_list: ["item1", "item2"] # List format example
- complex_structure: # Nested structure
name: "string"
value: 0
items: []
```

### LangChain Integration
- Auto-generate Pydantic models from variable definitions
- Use structured output parsers during model execution
- Handle parsing errors and validation failures

## Acceptance Criteria
- [ ] `vars` field parsing in YAML compiler
- [ ] Pydantic model generation from variable definitions
- [ ] LangChain structured output integration
- [ ] Variable access via dot notation
- [ ] Template resolution with structured data
- [ ] Type validation and error handling
- [ ] Documentation with examples

## Files to Modify
- `src/orchestrator/compiler/yaml_compiler.py` - YAML parsing
- `src/orchestrator/execution/variable_manager.py` - Structured storage
- `src/orchestrator/models/` - Structured output integration
- `src/orchestrator/core/pipeline.py` - Variable access API

**Priority**: High - Core data management requirement from #307