docs: add experiment framework design document

[abrichr]

Summary

• Reframes OpenAdapt as a general-purpose computer use framework, not a single-method system
• Defines experiment tracks: demo-conditioning, LoRA-per-task, GRPO, SFT, API baselines, UI-Venus base model
• Maps autoresearch pattern to OpenAdapt (mutable config, immutable eval, git-as-ratchet)
• Differentiates wright (Level 2: code changes) vs autoresearch (Level 1: config optimization)
• Tiered oracle architecture for trustworthy evaluation (static constraints → offline proxies → replay evals → live evals → adjudication)
• Multi-objective lexicographic scoring (not single scalar)
• Mutation surface ordering by signal-to-blast-radius (evaluator first, then narrow agent surfaces)
• Reproducibility requirements and experiment lineage
• Phased action plan: oracle quality → narrow agent optimization → broader search → training loops

Test plan

• Review doc for accuracy and strategic alignment
• Validate referenced codebase components exist

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