Feature/n agent support

[Raandom]

Description

It includes all the functionality to run full n agent benchmarks, using the already existing codebase for that setup in the repository.

Changes

src/cooperbench/eval/sandbox.py
Adds test_merged_n(), the N-agent counterpart to the existing test_merged(), which sets up N git branches and folds them together sequentially (agent2 into agent1, then agent3, etc.) before running each feature's test suite. Also adds the supporting helpers _setup_branches_n(), _merge_fold(), _build_merged_n_result(), and _merged_n_error_result() — all existing 2-agent functions are untouched.

src/cooperbench/eval/evaluate.py
Adds a team branch in _evaluate_single() that loads one patch per feature and routes to test_merged_n(), with dual-write of the legacy feature1/feature2/both_passed keys when N=2 for backward compatibility. Also updates the single-run display to handle the new features_result dict, and guards the GCP batch path against N>2 team runs.

src/cooperbench/runner/tasks.py
load_subset() now reads a "groups" key (N-element arrays) from subset JSON files alongside the existing "pairs" key, and discover_tasks() no longer hardcodes a 2-element destructure when iterating subset groups, making N-agent task definitions possible without touching existing subset files.

src/cooperbench/runner/core.py
All both_passed checks in _process_eval_result() and the inline eval progress display are updated to also check all_passed, so team eval results are correctly counted in run summaries alongside legacy coop results.

scripts/check_gold_conflicts.py
Generalises the existing pairwise gold conflict checker to support arbitrary N-tuple groups via a new --group-size N flag (default 2, fully backwards compatible). Adds discover_all_groups(), check_one_group(), _build_merge_script(), and write_subset_json(), and fixes two pre-existing Modal bugs: containers now start with sleep infinity to stay alive, and sb.terminate() is wrapped to tolerate already-terminated sandboxes.

tests/eval/test_sandbox.py
Adds TestMergedNErrorResult (schema correctness for the N-agent error result, including dual-write legacy keys for N=2) and TestTestMergedNSignature (source-level invariants verifying the sequential fold strategy, identical-patch short-circuit, lead-alone fallback, and N-branch loop are all present).

tests/eval/test_evaluate.py
Adds TestEvaluateSingleTeamRouting with five cases covering: team setting routes to test_merged_n and not test_merged; 2-agent team runs dual-write legacy keys; 3-agent team runs omit legacy keys; feature_ids are forwarded correctly; and coop runs are unaffected.

tests/runner/test_tasks.py
Adds TestGroupsSubsetKey with four cases verifying: load_subset() reads the "groups" key; "groups" takes precedence over "pairs" when both are present; discover_tasks() emits N-element feature lists from a groups subset; and the existing "pairs" key still works unchanged.

Checklist

• Code follows project style
• Tests pass locally
• Documentation updated (if needed)