fix: report submit registry failures

[caioribeiroclw-pixel]

Summary

• mark submit --json as failed when the registry reports a failed submission
• include registry errors in the JSON envelope
• exit non-zero for failed submit results instead of returning exit_code: 0

Why

While submitting caioribeiroclw-pixel/pluribus, the registry returned an HTML 429 response. skillfish submit --json surfaced the failure in failed[], but still returned success: true and exit_code: 0, which is risky for CI/agent automation.

Fixes #68.

Checks

• npm run typecheck
• npm test
• git diff --check

[caioribeiroclw-pixel]

Closing this in favor of maintainer PR #70, which addresses the same failure mode and is green in CI. I validated #70 locally from the reporter side and left the details there, so keeping this open would just split review attention.