Promote avg-strategy 1000iter to default.yaml; archive other configs

The avg-strategy 1000iter file (with the recent +traversals/+LR/+LCFR
changes) is the canonical "best-known" config. Renamed it to
default.yaml so users start from a single, obvious entry point and
override one field per ablation via --set. Other 12 configs moved to
configs/archive/ — kept for historical reproduction, not for active use.

- configs/deep_cfr/{default.yaml, smoke.yaml} are the only active configs
- experiment_name shortened to "deep-cfr-default" (was a long mouthful)
- AGENTS.md examples and Project Layout section rewritten around
  default.yaml; ablation example shows the override-one-field pattern
- Tests pointed at the archived slot-playability config for the legacy
  reproduction assertions

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/games/classic/test_deep_cfr_trainer.py

@@ -44,7 +44,7 @@ def test_deep_cfr_loads_smoke_yaml_config() -> None:
 
 
 def test_deep_cfr_loads_mapped_legacy_reproduction_config() -> None:
-    config = load_config("configs/deep_cfr/deep-cfr-selfplay-full-depth-slot-playability.yaml")
+    config = load_config("configs/archive/deep-cfr-selfplay-full-depth-slot-playability.yaml")
 
     assert config.run.experiment_name.endswith("slot-playability")
     assert config.run.seed == 79
@@ -92,7 +92,7 @@ def test_deep_cfr_train_cli_accepts_run_and_traversal_config_overrides() -> None
             ],
         },
     )()
-    config = load_config("configs/deep_cfr/deep-cfr-selfplay-full-depth-slot-playability.yaml")
+    config = load_config("configs/archive/deep-cfr-selfplay-full-depth-slot-playability.yaml")
 
     overridden = _with_overrides(config, _train_overrides_from_args(args))