Why: The model A/B branch merged into main via #296 (the final item of the
quality improvement plan); its task card must be backfilled and archived to
prevent bookkeeping debt.
Evidence: SELF - merge 63a2485 verified via git log --merges against
origin/main; card backfilled with the PR link.
Impact: .task/feature-agent-model-ab.md moved to .task/archive/ with status
backfilled; no code or submodule change.
Refs: task.sh close semantics (replicated from a temp worktree because the
main checkout is occupied by a parallel writer's branch).