Add offline submission bundle validator for miners

[claytonlin1110]

Summary

• Closes Add local submission bundle validator for miners #3: adds an offline CLI (python scripts/validate_submission.py / python -m trinity.validate_submission) that checks submissions/final_model/ before miners open a PR.
• Validates required files, best_theta.npy shape against ParamSpec.n_total (13312), finite floats, and coherent summary.json; reports optional history.json / eval.json.
• Documents the checker in CONTRIBUTOIN.md, README submit workflow, and submissions/final_model/README.md; unit tests cover pass/fail cases.

Test plan

• python -m pytest tests/test_validate_submission.py -q (8 passed)
• Run python scripts/validate_submission.py against a real experiment best_theta.npy + summary.json
• Confirm a deliberately broken bundle exits non-zero with clear errors

[tmimmanuel]

1. Add the offline validator step to PR automation before submission upload. Right now the workflow still only checks for best_theta.npy, so invalid bundles can still get queued and fail later. Run python scripts/validate_submission.py --dir pr/submissions/final_model in the workflow and fail early if it exits non-zero.

2. Move validate_submission.py out of scripts/ into a utility path, like utility/validate_submission.py. The current scripts/ classifier will keep tagging that file as train/eval, which creates label noise for a tooling-only change.

[tmimmanuel]

• CONTRIBUTOIN.md and src/trinity/validate_submission.py still present python -m trinity.validate_submission as a supported command. On a fresh clone, that fails unless PYTHONPATH=src is set or the package is installed. The zero-config entrypoint is the new wrapper in utility/validate_submission.py, so the docs should either drop the module form or make the PYTHONPATH requirement explicit.