docs: add LoRA-per-task design document (#54)

Literature-backed design for task-specific LoRA adapters with runtime
routing. Covers architecture, training pipeline, data collection
(including correction flywheel as training data source), update
economics, and validation plan. Positioned as one experiment track
within the broader OpenAdapt experimentation framework.

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

Author: abrichr