Improve monthly execution review diagnostics

[Pigbibi]

Summary

This PR improves the downstream monthly execution review package for BinancePlatform.

• add zero-trade diagnostics grouped by strategy sleeve and gate so no-trade months are easier to audit
• avoid duplicated Claude Primary Review headings when the primary review markdown is re-rendered with a secondary review
• tighten the auto-optimization completion guard so old generic gating summaries are not mistaken for the new zero-trade diagnostics
• document the auto optimization and guarded auto-merge workflow boundaries

Validation

• python3 -m pytest tests/test_monthly_report_bundle.py tests/test_render_monthly_ai_review.py tests/test_prepare_auto_optimization_pr.py

Refs QuantStrategyLab/AuditOrchestrator#1