Auto-derive run dir from experiment_name + timestamp

Drop checkpoint.directory from config — config defines what an experiment
is, not where its outputs go. The CLI now computes the run directory from
run.experiment_name plus a timestamp, defaulting to runs/tmp/ for
throwaway runs and runs/ when --keep is passed.

- Remove CheckpointConfig.directory and DeepCFRConfig.checkpoint_path
- DeepCFRTrainer takes run_dir: Path explicitly
- CLI: add --keep boolean; --resume requires an explicit path (no shortcut)
- Auto path: runs/[tmp/]<YYYY-MM-DD_HHMMSS>_<experiment_name-kebab>/
- Rename 13 configs to kebab-case; strip directory: lines; kebab their
  experiment_name values
- Rewrite AGENTS.md training/run sections; document
  archive/tmp/<flat> layout, --keep, kebab-case scope
- Update tests for new run_dir flow and dropped --resume shortcut

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

configs/deep_cfr/smoke.yaml

@@ -1,4 +1,5 @@
 run:
+  experiment_name: smoke
   max_iterations: 1
   seed: 1
   device: cpu
@@ -25,7 +26,6 @@ memory:
   strategy_capacity: 1000
 
 checkpoint:
-  directory: runs/deep_cfr/smoke
   save_every: 0
   save_latest: false