[Feat] implement ModelEvaluator for automated onboarding flow (#1153)

[aby42]

This PR implements the initial ModelEvaluator (OnboardEvaluate) as the first milestone for the model onboarding procedure.

To keep the discussion centralized, I have posted the detailed architectural plan, roadmap, and sample JSON output directly in the issue comments.

Please refer to the discussion here: #1153 (comment)

Key Changes

Added src/training/model_eval/onboard_eval.py with core benchmarking logic.

Supported reasoning (ARC, MMLU-Pro) and performance metrics tracking.

This is a Draft PR intended for early architectural review of the code structure.

FIX #1153

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