Reject non-boolean child gate results

[EauDoon]

Summary

• require literal booleans in zero-exit decide and prove payloads
• fail closed for string, number, array, null, missing, and malformed success fields
• preserve existing nonzero-exit and fallback behavior

Tests

• focused stack suite 19/19
• full npm run check 21/21
• syntax, GUI smoke, and diff checks

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.