fix(argo): mount training-data read-write in fxcache step

The imbalance bar cache now writes to <mbp10_dir>/.cache/ on the PVC.
The ensure-fxcache step needs write access to persist the cache
across runs (was readOnly: true, causing silent cache-write failures).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

infra/k8s/argo/train-multi-seed-template.yaml

@@ -360,7 +360,7 @@ spec:
         volumeMounts:
           - name: training-data
             mountPath: /data
-            readOnly: true
+            readOnly: false
           - name: feature-cache
             mountPath: /feature-cache
         args: