Add documentation for YAML schema validation implementation

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Co-authored-by: ilaflott <6273252+ilaflott@users.noreply.github.com>

Author: Claude

YAML_SCHEMA_VALIDATION.md

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+# YAML Schema Validation Implementation
+
+## Overview
+
+This document describes the implementation of YAML schema validation for fremorizer, which eliminates the hard dependency on `fre-cli`'s `yamltools` module while maintaining backward compatibility.
+
+## Issue Context
+
+The original issue requested a "YAML approach with schema validation, no fre-cli". The goal was to:
+1. Remove the hard dependency on `fre-cli` for YAML processing
+2. Implement robust schema validation for YAML configuration files
+3. Maintain backward compatibility with existing workflows
+
+## Implementation
+
+### New Modules
+
+#### 1. `fremorizer/cmor_yaml_schema.py`
+Defines the JSON schema for CMOR configuration and provides validation functions.
+
+**Key functions:**
+- `get_cmor_schema()`: Returns the complete JSON schema definition
+- `validate_cmor_yaml(config_dict)`: Validates a configuration dictionary against the schema
+
+**Schema features:**
+- Validates all required fields (mip_era, directories, exp_json, table_targets)
+- Enforces MIP era enum (CMIP6, CMIP7)
+- Validates year formats (YYYY pattern)
+- Ensures complete gridding dictionaries when present
+- Supports optional fields (start, stop, calendar_type, freq, gridding)
+
+#### 2. `fremorizer/cmor_yaml_consolidator.py`
+Provides native YAML loading and validation functionality.
+
+**Key functions:**
+- `load_and_validate_yaml(yamlfile, ...)`: Loads and validates a YAML file
+- `_expand_variables_in_config(config)`: Expands environment variables recursively
+
+**Features:**
+- Single-file YAML loading
+- Automatic schema validation
+- Environment variable expansion (e.g., `$HOME`, `${VAR_NAME}`)
+- Optional output file generation
+- Compatibility parameters for fre-cli signature
+
+**Limitations compared to fre-cli:**
+- Does not support multi-file YAML consolidation
+- Does not support platform/target/experiment-specific overrides
+- No variable substitution beyond environment variables
+
+### Modified Modules
+
+#### `fremorizer/cmor_yamler.py`
+Updated to use native YAML loading by default, with automatic fallback to fre-cli if available.
+
+**Changes:**
+- Lines 27: Added import for `load_and_validate_yaml`
+- Lines 96-111: Replaced hard dependency with conditional logic:
+  - If `fre-cli` is available: Use `consolidate_yamls` (advanced features)
+  - If `fre-cli` is NOT available: Use native `load_and_validate_yaml`
+  - No more ImportError - both paths work
+
+### Dependencies
+
+#### Added to `pyproject.toml`:
+```python
+'jsonschema',  # For YAML schema validation
+```
+
+#### Added to `environment.yaml`:
+```yaml
+- conda-forge::jsonschema
+```
+
+### Tests
+
+#### `fremorizer/tests/test_cmor_yaml_validation.py`
+Comprehensive test suite covering:
+- Schema retrieval and structure
+- Valid configuration acceptance
+- Missing required field rejection
+- Invalid enum value rejection
+- Invalid year format rejection
+- Optional field handling
+- YAML file loading
+- File not found errors
+- Invalid YAML syntax errors
+- Schema validation failures
+- Environment variable expansion
+- Output file generation
+
+**Test results:** All standalone tests pass successfully.
+
+### Documentation
+
+#### Updated `README.md`:
+- Added `jsonschema` to requirements
+- Added "YAML Processing" section explaining:
+  - Native YAML loading with validation
+  - Automatic fallback to fre-cli if available
+  - Instructions for installing fre-cli for advanced features
+  - Note about limitations of native loader
+
+#### Created `example_cmor_config.yaml`:
+- Demonstrates complete YAML structure
+- Includes inline comments explaining each field
+- Shows both required and optional fields
+- Provides examples for CMIP6 and CMIP7
+
+## Backward Compatibility
+
+The implementation maintains 100% backward compatibility:
+
+1. **With fre-cli installed:** Uses `consolidate_yamls` (no change in behavior)
+2. **Without fre-cli installed:** Uses native loader (new functionality)
+3. **Existing tests:** Continue to work with mocked `consolidate_yamls`
+4. **Configuration format:** Exactly the same structure expected
+
+## Usage Examples
+
+### Basic usage (native loader):
+```bash
+fremor yaml model.yaml --exp test --platform ncrc4 --target prod
+```
+
+### With advanced features (requires fre-cli):
+```bash
+pip install fre-cli
+fremor yaml model.yaml --exp test --platform ncrc4 --target prod
+```
+
+### Validating a YAML file:
+```python
+from fremorizer.cmor_yaml_consolidator import load_and_validate_yaml
+
+config = load_and_validate_yaml('model.yaml')
+# Raises ValueError if invalid
+```
+
+## Migration Path
+
+For users currently using fre-cli:
+1. No changes required - fre-cli continues to work
+2. Optional: Remove fre-cli dependency for simpler workflows
+3. Optional: Update YAML files to be standalone (no multi-file inheritance)
+
+For new users:
+1. Install fremorizer: `pip install fremorizer`
+2. Create YAML config using `example_cmor_config.yaml` as template
+3. Run: `fremor yaml your_config.yaml`
+4. No fre-cli installation needed
+
+## Future Enhancements
+
+Potential improvements:
+1. Multi-file YAML consolidation in native loader
+2. Platform/target/experiment override support
+3. Enhanced variable substitution (beyond environment variables)
+4. YAML schema auto-generation from Python types
+5. Additional validation rules (path existence, value ranges)
+6. Schema versioning for CMIP6 vs CMIP7 differences
+
+## Testing Notes
+
+The full test suite requires the CMOR library, which is not available in all environments.
+Standalone tests confirm:
+- Schema validation works correctly
+- YAML loading and validation works
+- Environment variable expansion works
+- Backward compatibility is maintained
+
+CI tests will run the full suite in a proper conda environment with all dependencies.