Fix submit_kernel non-zero exit for failed submissions

[fallintoplace]

PR description

Make submit_kernel.py return a non-zero exit code when a Kaggle submission or evaluation fails, so CI and agents do not treat a broken submission as a successful run.

The root cause was that the script only exited non-zero when kernel execution itself failed. Submission failures from competition_submit_code or non-complete evaluation results were printed, but the process still returned success.

This change keeps track of an overall success state across kernel execution, submission acceptance, and evaluation polling, then exits with status 1 whenever any of those stages fails.

Checklist

• Tests pass (uv run pytest)
• All commits are signed off (DCO) using git commit -s
• License header (SPDX-FileCopyrightText / SPDX-License-Identifier) is included on new non-markdown code files
• The change stays focused on one workflow where possible (see CONTRIBUTING.md)
• Credentials are read from KAGGLE_API_TOKEN (or another env var) and no account artifacts are committed

Not applicable for this PR:

• No new workflow was added or renamed, so workflow markdown, README workflow table, and migration notes did not need changes.
• No plugin metadata changed, so claude plugin validate . was not required.

Validation

• uv run pytest

[alessiodevoto]

Hi @fallintoplace, could you fix the body of your PR that is not legible as is now? Also, we have updated the contributing guidelines, so please rebase on main and make sure to follow those from the next commit! Thanks 🙂

[alessiodevoto]

LGTM!