fix(harbor): keep reward output numeric

[SergioHC95]

Summary

The Harbor verifier captures the metric parser with 2>&1, so a malformed metrics.json folds the parser diagnostic into ACCURACY and writes a non-numeric reward.txt.

This keeps stdout exclusively numeric while preserving parser diagnostics in metrics_parse_error.txt and the verifier stderr stream. A genuine score of zero and a parse failure remain distinguishable in verifier artifacts.

Validation

bash -n src/harbor_adapter/template/tests/test.sh
git diff --check

Refs #8