Add YAML configuration file support to CLI

[devin-ai-integration]

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Summary

This PR adds YAML configuration file support to the llama.cpp CLI while maintaining 100% backward compatibility with existing command-line arguments. Users can now specify configuration via --config path/to/config.yaml, with CLI arguments taking precedence over YAML values when both are provided.

Requested by: @jakexcosme
Devin session: https://app.devin.ai/sessions/9cd38b75d2344ac8a64126e2e01adc92

Key Changes

• New --config argument: Accepts YAML configuration files
• yaml-cpp integration: Added via FetchContent for robust C++ YAML parsing
• Comprehensive YAML structure: Supports all major parameter categories (model, sampling, speculative, etc.)
• CLI precedence: Command-line arguments override YAML configuration values
• Extensive test suite: 9 new test cases covering functionality and backward compatibility

YAML Configuration Example

n_predict: 100
n_ctx: 2048
prompt: "Hello, world!"
model:
  path: "model.gguf"
sampling:
  seed: 42
  temp: 0.8
  top_k: 40
  top_p: 0.9
speculative:
  n_max: 16
  p_split: 0.1

Test Results

All 41 tests pass (100% success rate), including:

• 6 YAML functionality tests (basic parsing, CLI override, error handling, complex structures)
• 3 backward compatibility tests (CLI without YAML, equivalent YAML vs CLI, major CLI options)
• All existing tests continue to pass

Areas for Review

🔴 High Priority:

1. Dependency build verification: Ensure yaml-cpp builds correctly across platforms
2. CLI precedence testing: Verify CLI arguments properly override YAML values in all scenarios
3. Error handling: Test with malformed YAML files and missing files
4. Backward compatibility: Confirm no regressions in existing CLI functionality

🟡 Medium Priority:
5. YAML structure accuracy: Verify YAML parameter names match CLI argument names
6. Performance impact: Assess startup overhead from YAML parsing
7. Security considerations: Review YAML parsing for potential security issues

🟢 Low Priority:
8. Code organization: The YAML parsing functions are quite long and repetitive
9. Documentation: Verify help text and examples are clear

Implementation Notes

• YAML parsing occurs before CLI argument parsing to allow proper override behavior
• Uses yaml-cpp 0.7.0 for robust C++ YAML handling with comprehensive error reporting
• Maintains existing argument parser structure with minimal modifications
• Comprehensive error messages for YAML parsing failures

The implementation prioritizes backward compatibility and follows existing code patterns in the llama.cpp codebase.

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