refactor(prover): separate outcome evaluation from rendering

[eviehoward]

Summary

Moves exit-code logic out of the renderers and into prove(), so outcome evaluation happens once in a single place rather than independently in each renderer.

Related Issue

Closes #2411

Changes

• render_compact and render_report return () instead of i32; all exit-code logic removed from both
• prove() computes exit code after rendering: i32::from(findings.iter().any(|f| !f.accepted))
• Call site in prove() drops unused policy_path/credentials_path args from render_compact
• Both renderer banners gate on active.is_empty() rather than path_count > 0, so terminal output and exit code agree for pathless unaccepted findings
• Four unit tests added covering: empty findings, all-accepted, unaccepted with paths, pathless unaccepted (the case the old renderers got wrong)

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated - not applicable, no infrastructure, sandbox, or policy changes

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated - not applicable, internal refactor, no behavioral or API change

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