fix: require independent dual AI for candidate readiness

[Pigbibi]

Summary

• remove the single-reviewer early-return path
• require both independent LLM reviews before a proposal can become candidate-ready
• treat missing independent review as an escalation, never an approval

Safety

This tightens review controls. It does not add execution authority.

Validation

• lifecycle reviewer tests: 10 passed
• targeted Ruff check passed
• git diff check passed