docs(prover): correct prove() exit code doc comment

[eviehoward]

Summary

Corrects the prove() doc comment which incorrectly described exit code 1 as
firing on "critical or high findings." The prover has no severity grades — exit
code 1 fires on any unaccepted finding.

Related Issue

Closes #2394

Changes

• Corrected exit code descriptions in crates/openshell-prover/src/lib.rs:31-32

Testing

• mise run pre-commit passes
• Unit tests added/updated — not applicable, doc comment only
• E2E tests added/updated — not applicable

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated — not applicable

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[github-actions]

All contributors have signed the DCO ✍️ ✅
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[eviehoward]

I have read the DCO document and I hereby sign the DCO.

[matthewgrossman]

recheck