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>

configs/deep_cfr/default.yaml

@@ -1,5 +1,5 @@
 run:
-  experiment_name: lost-cities-deep-cfr-opponent-average-strategy-512x3-1000iter
+  experiment_name: deep-cfr-default
   seed: 79
   max_iterations: 1000
   max_minutes: null