[codex] Tolerate protected-branch push rejection in theme snapshot

[Pigbibi]

Summary

• keep Theme Momentum Snapshot artifact output successful when branch protection rejects direct push to main
• continue failing on non-ruleset push errors
• add a workflow regression check for the protected-branch path

Root Cause

The latest scheduled run failed at Commit snapshot because GitHub repository rules rejected a direct push to protected main with GH013 required status checks.

Validation

• gh run view 28706716383 --repo QuantStrategyLab/ResearchSignalContextPipelines --log-failed
• uv run --with pytest pytest tests/test_theme_momentum.py tests/test_theme_momentum_snapshot_workflow.py tests/test_dispatch_shadow_signal_workflow.py -q
• actionlint -oneline .github/workflows/theme_momentum_snapshot.yml
• git diff --check

[github-actions]

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