fix: make 'best' parent-selection mode reachable from CLI and sort by highest score

[mvanhorn]

Summary

Fix the argparse choices list in DGM_outer.py by adding the missing comma so it reads ['random', 'score_prop', 'score_child_prop', 'best']. In choose_selfimproves(), add reverse=True to the sorted(...) call in the elif method == 'best': branch so the highest accuracy_score candidates are chosen first. Add a new root-level test_dgm_outer.py (mirroring the existing root-level test_swebench.py convention; the repo has a pytest.ini) with a unit test that calls choose_selfimproves(..., method='best') against synthetic candidate metadata (three fake commits with distinct scores, using tmp_path-backed metadata or monkeypatched loading as the function's data access requires) and asserts the top-scoring parent is selected first, plus a test that the argparse parser accepts both best and score_child_prop.

Why this matters

DGM_outer.py has two related bugs in the best parent-selection path. First, the argparse choices list for --choose_selfimproves_method is written as ['random', 'score_prop', 'score_child_prop' 'best'] - the missing comma makes Python concatenate the adjacent string literals into 'score_child_propbest', so best (and an explicitly passed score_child_prop) are rejected by the CLI. Second, the method == 'best' branch in choose_selfimproves() sorts candidates by accuracy_score in ascending order and takes the first N, so it selects the lowest-scoring parents instead of the highest-scoring ones. Because parent selection is the search pressure in DGM, an inverted best strategy silently corrupts any experiment comparing selection strategies.

See #31.

Testing

• Happy path: choose_selfimproves(..., method='best') with three synthetic candidates scoring 0.2/0.5/0.9 and selfimprove_size=1 returns the 0.9-score commit. - selfimprove_size=2 returns the top two commits in descending score order. - selfimprove_size larger than the candidate count returns all candidates without error. - CLI parser accepts --choose_selfimproves_method best and --choose_selfimproves_method score_child_prop; an invalid value (e.g.

Fixes #31