docs: add experiment framework design document (#121)

Frames OpenAdapt as a general-purpose computer use framework with
multiple experiment tracks (demo-conditioning, LoRA-per-task, GRPO,
SFT, API baselines, UI-Venus base model). Covers autoresearch pattern
adaptation, wright+autoresearch composition, tiered oracle architecture,
multi-objective scoring, mutation surface ordering, and reproducibility
requirements.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: abrichr